First, is that yes, there are stupid people out there, doing illegal things, and posting them on U tube/Facebook, what have you. Some of them even wind up getting caught.
Not too long ago there was a case of a model and some pictures of her with some guns. Turns out she had been busted (the legal kind), about drugs or something, and parole/probation = no guns. Pics on the net of her violating her probation, go back to jail. Do not pass go, do not collect $200.
Second thing, UTube videos can come from anywhere. THere are still many places in the world where you can legally convert any gun you want for a price. So you might see things on U tube that are a crime in the US, but not a big deal where the video was made.
Personally, I think its more likely that its stupid people thinking they can show off and not get caught. But thats just my opinion.