Poppers with amyl and other chemicals are "illegal" in a lot of countries now.
Hence the different formulations now using alternate stuff.
Its no worse than the public being exposed to weed, tobacco alcohol.
People like to get fucked up to escape the everyday shit..
As far as the popper vids online, I did my first poppers just before all that Pornhub news.
I was so into the popper trainer vids I downloaded all I could find; but a month later most sites deleted them.
I really cant see why; they are just porn clips with the odd shot of someone smelling poppers.
The vids are actually way better doing poppers, the music works well when your in a high horny frenzy!
I found 180 different popper vids..some are re-edits.
So my long winded answer to the question..
"Is there some issue with popper porn that porn sites don't like?"
Yes they have some "moral issue with it" ,
but hey if you like abusive porn, people taking a shit on someone else, or rape porn well NO PROBLEM.
Go figure.