The most revealing thing about Friday’s Union Square pop-up riot is that as police dispersed the mob, members started chanting, “Black Lives Matter.”

Make no mistake: This was not a protest on the part of teenagers drawn to 14th Street by Kai Cenat, an online “influencer” with millions of followers, including thousands eager for free PlayStations he used to lure them to demonstrate his importance.

(He faces incitement to riot charges.)

But as dozens in the crowd were being arrested for vandalism, they realized they had a useful line of defense: invoking the movement predicated on police mistreatment of minority youth.

The NYPD was not intimidated into turning a blind eye toward the destruction of food carts and cars — not this time around.

But the chant pulled up the curtain on what has long been the dirty secret of mass protests, from the Watts riots to the rampages accompanying the “protests” of the police homicides of George Floyd in Minneapolis and Michael Brown in Ferguson, Mo.

There’s always been a significant part of the crowd that is, in Harvard social scientist Edward Banfield’s legendary phrase, “rioting for fun and profit.”

Banfield, in his landmark 1970 book “The Unheavenly City,” usefully delineated mass actions.

“Demonstrations” linked clearly to a cause are not riots — and Banfield defended them.

So, too, did he exhibit sympathy for the “outburst of righteous indignation” a specific incident sparked by concern about its “wrongfulness going unpunished.”

The George Floyd-linked actions were stirred by such concerns, at least at first.

But Banfield also had a distinct diagnosis of events such as those in Union Square and a similar unruly pop-up last week in downtown Chicago: rampages. “Young men are naturally restless, in search of excitement, thrills, ‘action,’” he wrote.

The young male “wants dramatic reassurance that he ‘can make things happen’ and breaking the law is one of the few actions that immediately and demonstrably makes things happen.”

Rioting “is a way of making them happen on a wholesale scale.”

The link, then, between Union Square vandalism and gang membership and violence is clear.

Such rampages, observed Banfield, “occur today not only in the slums but elsewhere” — and need not have any link to race or racial justice.

Miami authorities who must gird each year for drunken spring-break revelers’ violence can vouch for Banfield’s analysis.

When police seek to arrest those “committing acts of vandalism and harassing the police and an officer has to arrest a drunk disturbing the peace, the youngster will often set upon the policemen and a major riot looms before reinforcements can be called.”

New York is fortunate the NYPD interrupted just that scenario by making a Level 4 call for 1,000 police to swarm Union Square. But we’ve seen it go the other way too many times.

These rampages can also blur into a “foray for pillage,” Banfield wrote.

“Here the motive is theft, and here also boys and young adults of the lower class are the principal offenders. Stealing is ordinarily most conveniently done in private, of course, but when disasters — fires, floods, power failures — interrupt law enforcement it may be done as well or better in public.”

That’s exactly what happened as the 2020 George Floyd protests spun out of control — and some 450 New York businesses were damaged or destroyed.

In Floyd’s home city, Minneapolis, minority business owners, many law-abiding immigrants, lost all they’d worked for in a flash (mob).

We have a serious problem distinguishing between riots and legitimate protest — as made clear when Chicago Mayor Brandon Johnson last week objected to Michigan Avenue rampagers being characterized as a “mob” — which is what it was.

(By the way, what do all those kids do when they’re not on their phones waiting to join the next flash mob?)

Too often, legitimate protests provide pretext and cover for riots aimed at fun or profit.

That’s exactly why those stragglers around Union Square fell back on the Black Lives Matter chant — to excuse their behavior. But the effort was late and transparent.

During the George Floyd protests, however, it worked — to the point that the NYPD, in trying to curtail violence and property damage those marches enabled and devolved into, became the legal villain.

The city settled a lawsuit, paying $13 million to those whose rights the NYPD allegedly violated — as if it could be clear in the moment who was protester and who the rioter.

We are far too quick to ascribe a political motive to indefensible riots.

Let’s be clear: A very thin line separates Kai Cenat from Al Sharpton. The influence of both has been evident on New York’s streets.