Except you'd have no way of knowing whether they knew they were violent rioters.

What you're calling kidnapping is actually referred to as arresting, and it is entirely possible to identify violent individuals with certainty. Every riot in Seattle has had multiple police helicopters circling the area; it isn't difficult to identify and track particularly violent participants.

Proving whether that person participated in the riot is a matter for the courts, all that's needed for an arrest is reasonable suspicion.

> There is never, ever, ever any possible excuse for kidnapping people without due process.

I don't know if you are a native speaker, but the word kidnap might not be the one you should be using. "Kidnap" has strong connotations, and while sime people will happily use that property of the word to create feelings I don't think it is appropriate here.

kidnap: abduct (someone) and hold them captive, typically to obtain a ransom.

There's a lot to be said about police, federal agents, TSA etc, but lets keep this serious.