Yet it is one thing to say a company has no control over the conduct of online commenters, and another when its users are in people’s homes or cars. Airbnb, like others, has been forced to learn the limits of its status as a platform. In response to reports of renters’ damaging and ransacking homes, it added a round-the-clock hotline for people in unsafe situations and a policy covering $1 million in loss or damages.

The second web business principle is to minimize the number of paid on-staff employees. Tech companies have long shunned the idea of hiring lots of sales staffers or call-center workers. Instead they automate ad sales with auction algorithms or offer help forums where other customers offer advice on their sites. When Instagram was acquired by Facebook, it employed 13 people; Kodak, in its heyday, employed more than 140,000.

That mentality may be why new on-demand companies are running into trouble with workers. Most of these companies avoid having employees by using contract workers. But some are wondering whether the companies are pushing the definition of contract worker too far. Uber drivers have filed class-action lawsuits in Massachusetts and California, and advocates are pushing for things like benefits and disability compensation for workers at many start-ups.

The third related principle is to automate everything — whether selling ads, flagging inappropriate content or assessing employee performance. That notion also meets its limits in the real world. When Brian Chesky, the co-founder and chief executive of Airbnb, responded to complaints about vandalism, he emphasized that Airbnb had “algorithms that identify suspicious behavior.” That’s nice, yet algorithms, when people’s safety and well-being are involved, are not enough. Airbnb said it combined automation and human involvement with 100 people who reviewed suspicious activity.

The belief that problems can be solved without involving people is probably why many of these companies did not meet with regulators and officials before starting services in new cities. And it has come back to haunt them. Luther Lowe, director of public policy at Yelp, had some basic advice for Uber that could apply to Airbnb, Lyft and others: Hire a lobbyist and meet with the mayor and the city council before setting up shop.

Uber seemed to put that lesson into practice in August when it hired David Plouffe, an expert in grass-roots campaigning and the manager of President Obama’s winning run in 2008, as a senior policy and strategy executive. “Our roots are technology, not politics,” Travis Kalanick, Uber’s co-founder and chief executive, wrote about the hire. “The result is that not enough people here in America and around the world know our story, our mission and the positive impact we’re having.”