SANTA CLARA — The customer is always right. Except when the customer has been tailgating for three hours followed by regular visits to stadium beer stands. Then the customer is frequently stupid.

I think that’s the primary take-away from Sunday’s borderline appalling and bizarrely sad moment at Candlestick Park. Although something else about the current pro sports culture might have been at work, as well.

Here’s what happened: In the fourth quarter, Arizona defensive end Calais Campbell was down on the turf and injured seriously enough that a motorized cart was summoned to carry him off. During the delay, with players from both teams gathered in concern around Campbell, the crowd of mostly 49ers fans decided to pass the time by organizing … the wave!

As various sections or people began standing up and down and cheering, reaction from Cardinals and 49ers players on the field was one of incredulity. Several 49ers players gestured with their arms to stop. Ted Robinson, the team’s radio play-by-play man, railed against the scene on the air, calling it “disrespectful” and “wrong.” He asked anyone in the stadium who was listening to please quit waving.

Some did.

Many didn’t. It’s probably the NFL’s fault. Maybe this disclaimer needs to be printed on all tickets: DURING THIS GAME WHEN IT APPEARS THAT A PLAYER MAY HAVE BEEN PARALYZED AND IS UNDERGOING MEDICAL TRIAGE, THE PROPER SPECTATOR REACTION IS TO NOT JOYOUSLY PERFORM AN INANE CROWD STUNT FROM THE 1980s.

Fortunately, after Campbell left Candlestick and was taken to an emergency room, word arrived that his injury was not as grim as it appeared. Feeling returned to his extremities, and he was able to return to Phoenix. But that doesn’t excuse the Candlestick fans’ senseless reaction. Monday, the incident came up briefly at 49ers coach Jim Harbaugh’s media conference. He was careful in outlining his thoughts and advice, but it’s clear where his sentiments fell.

“I don’t tell people what to do,” Harbaugh said. “But, I think people know that if the cart’s coming out on the field that signifies a pretty serious injury. And the best thing to do is silence, let the doctors do their work. And a collective prayer would be much appreciated.”

What the heck is going on out there in the chair-backed seats these days, anyway? Questionable fan behavior is not a totally new phenomenon. It’s been around since the NFL was playing in Canton and Decatur. But for the most part, that amounted to one or two knucklehead drunks starting a fight or throwing a drink on someone.

Oh, it’s true that back in 1980, a Raiders home crowd famously applauded when quarterback Dan Pastorini broke his leg because he could be replaced by fan favorite Jim Plunkett. But that was so out of the norm that it made big headlines. These days, it seems to be a weekly feature.

Or didn’t you hear what happened Sunday in Houston? A number of home fans decided to cheer lustily when their own starting quarterback, Matt Schaub, injured his ankle on a sack in the third quarter of the Texans’ 38-13 loss to St. Louis.

And in Denver, a profoundly weird thing occurred. The Broncos are the NFL’s best team at the moment. But the Mile High crowd unleashed some hearty boos when quarterback Peyton Manning, against huge underdog Jacksonville, allowed the last 15 seconds of the clock to expire at the end of the first half rather than run another play.

At the time, the Broncos were leading by two points and were on their own 20-yard line. They were also receiving the second-half kickoff — which meant they’d get the ball in roughly the same position at the start of the third quarter (and in fact drove for a touchdown when that happened, en route to a 16-point victory). But the crowd was apparently too besotted for logic to matter.

Or if not besotted, successfully marketed. In trying to figure out the situation, that’s part of my theory. Although the league has issues with alcohol every week, not every single person in every stadium is full of beer. Some are entirely sober. But they’ve become intoxicated in a different fashion.

The NFL has become the most successful sports league in North America with a marketing strategy that treats the game as entertainment as much as anything else. The smashing highlights, the emphasis on personalities as superheroes, the fantasy-league catering … it explains why NBC’s “Football Night in America” Sunday broadcasts often post the week’s highest ratings of any programming.

However, along the way, that strategy might have given too many customers the idea that they are indeed watching entertainers (who can be snarkily insulted on tweets) or superheroes (with video-game powers and cartoonish attributes) rather than real human beings. Those of us who see players limp in and out of training rooms every week, ice bags covering various limbs, can attest otherwise.

This also could explain why, as more dire and forbidding information keeps coming to the fore about concussions and football, reaction from rank-and-file NFL fans seems to be more of a collective shrug than shock. Ask anyone in my business. On a daily basis, more people ask which players to start on their fantasy teams than ask about whether a player’s concussion might cause long-term damage.

After Sunday’s sorry scene in Houston when Schaub’s injury drew approving shouts, Andre Johnson told reporters: “For them to cheer about something like that, it just shows they have no class.”

Actually, it might just show they are NFL fans in 2013.

Contact Mark Purdy at mpurdy@mercurynews.com. Follow him on Twitter at twitter.com/MercPurdy.