Oppressive, overcrowded New York is not much fun in the summer, but among theater-goers one institution makes the sweltering months worth waiting for. Shakespeare in the Park begins its 50th season this week, and in the past half-century the festival, managed by the Public Theater, has matured from an improbable experiment into a city birthright. Audiences are still filling the Delacorte Theater in Central Park, sited perfectly amid the tress and the water.

And amazingly, the tickets are still free – though you may have to get in line at daybreak, or even earlier.

Oskar Eustis, the Public's artistic director, is a broadminded, sometimes radical producer who, in recent seasons, has programmed Shakespeare in the Park with an eye to American politics and society. Twelfth Night, in 2009, spoke to the country's debate over marriage equality; The Merchant of Venice, in 2010, attacked the greed that led to our "lesser depression"; and Measure for Measure last year dug deep into our politicians' sexual hypocrisy.

But this summer, he's chosen As You Like It – with an original score by, of all people, Steve Martin. While Shakespeare's great pastoral does have a bit of bite, it's not the choice I would have expected for this post-Occupy, pre-election summer. (Coriolanus might have been the right call.) What's more, Eustis has paired As You Like It with another song-filled forest adventure: the fairytale musical Into the Woods, which is charming but not exactly rabble-rousing.

We'll see how things go. But in this anniversary season, there is already something to worry about. More and more, Shakespeare in the Park is tending away from the bedrock principle on which it was founded: that all tickets should be free. Joe Papp, the secular saint who fought for years to build a theater in Central Park, was able to rely mostly on the city government and a collection of munificent New Yorkers to fund his new theater. These days, however, Shakespeare in the Park receives its primary support from large corporations such as Bank of America and Bloomberg – and they want to be thanked.

Although the Public is cagey about just how many tickets go to corporate sponsors, its former executive director said that, on an individual night, the percentage of seats allocated to donors can hit 30%. On its website, the Public is not shy about the exchange rate between donations and tickets. "Entertain clients or employees this summer in Central Park!" it cheerfully proposes, and the more you give, the more seats the ushers will rope off for you. Not only that, the house will provide a convenient "VIP entrance" for you and your colleagues, in case you were worried you might have to mix with the lowly bardolators who camped out all night. And naturally, you should feel free to take the gift off your taxes – since this isn't a purchase, this is charity.

Individuals, too, can jump the queue for a price. This season, it's $175 to become a "summer supporter", which gets you a reserved seat and a few other nice benefits. (And, again, it's tax-deductible – so if you're itemizing next year, the tickets might actually cost you nothing!) It's funny, since scalping tickets for the Delacorte angers New Yorkers so much that Andrew Cuomo, the state attorney general in 2010, actually took time out of his gubernatorial run to shut down online touts. He handed the Public a small PR coup that year – but quietly, the theater kept selling tickets on its own.

It's easy to call this a necessary evil. A theater where 70% of tickets are free – and Joe Papp would surely agree here – is better than one where 100% are not. And I certainly don't begrudge the Public the money. Unlike Europe's leading houses, like the National Theatre in London or the Royal Dramatic Theater in Stockholm, the Public has to do all the work of a national producer with none of the government funding.

But the establishment of a market for tickets – this much "charity" for this many seats – does more than just let a few well-off people pay with money instead of time. It's much more harmful than that. It cheapens the entire institution, and it erodes the thing that makes it uniquely valuable: its openness. When Papp began his push for a permanent home for what was then called the New York Shakespeare Festival, he had to fight for years with city authorities, which insisted he charge admission. But for him, even a quarter was too much.

"I am trying to build our theater on the bedrock of municipal and civic responsibility," Papp wrote in 1958. Free tickets were not some utopian fantasy – they were the means by which he could build a sustainable organization, removed from what Papp called "the chaotic gambling of show business". Free was actually safer than paid, because free meant that audiences and the city were committed for the long haul.

Would Shakespeare in the Park have survived for 50 years had tickets cost $5 or $10 in 1962? I suspect not, though I can't be certain. But what is unquestionable is that the institution would never have achieved its position as a uniquely civic theater, one where plays 400 years old can speak directly to American society. Papp made a commitment to a Shakespeare in American English when most other theaters were either avoiding his work entirely or doing it in a sub-Olivier jaw. He pioneered color-blind casting decades before it was commonplace: James Earl Jones played Othello here, but he also played King Lear.

But what made all that matter was the audience he assembled: younger, less experienced, and from across the city and the nation. At the Delacorte, Shakespeare not only spoke to America but looked like it.

The individuals who buy their way in are missing that civic component of Shakespeare in the Park. For them, a play outside is just another summer trifle. But the corporations whose logos adorn the programs and the posters are doing something more destructive. They are using the egalitarian values of Shakespeare in the Park to legitimate their own perpetuation of inequality, and in the process are destroying the very egalitarianism they came to feed upon. A democratic institution is not supposed to have a VIP entrance.

Shakespeare in the Park still has a relevance that few other arts institutions enjoy. But if Bank of America and their confreres have their way with it, it may recede from our central stage to just a bunch of poor players signifying nothing. I have great admiration for Oskar Eustis, whose heart is unquestionably in the right place. But if we want to guarantee another 50 years for Shakespeare in the Park, perhaps this is the summer to occupy Arden.