I'm looking for any sort of link to news about a Stanford student being expelled for having a video of shooting an AR-15. Google doesn't know anything about it, not do a variety of other search engines.

When we overgeneralize, exaggerate and make patently false claims that would be easily dismissed if we just did any research, we muddy the conversation. Continuing to pass along false information and present it as fact is as harmful as the cancel culture you are lambasting (which, yes, also oversteps often).

For the record, my best guess is that you're referencing a case at Fordham University where a student posted a picture of himself (on Instagram, not Facebook, for what it's worth) holding a AR15 with the hashtag #DontTreadOnMe and was visited by University members to insure that it wasn't intended as a threat directed against other students. He wasn't expelled, though he did receive some restrictions, including not being allowed on campus without prior approval until his graduation. It should be noted that all classes during this time were virtual in any case, due to COVID.

So there's certainly things to take issue with there, and I don't agree with Fordham's decision. But it's also not what you described. It's not an ex post facto treatment for something he did years ago, it's not at Stanford, it's not an expulsion, and it's not happening the the majority or even many of those who have photos with an AR15. And when either side overgeneralizes and acts like a one-off is the norm, it damages discourse and is harming in its own way. 

If you are, in fact, referencing the Fordham case where a student was restricted from campus after posting an AR15 on Instagram (as opposed to a Stanford case where a student was expelled for facebook video of an AR15), then it's a testament to how one-off stories are identified, modifed and exaggerated in order to have the most visceral impact before being blindly passed on. So you asked why I rolled my eyes, and that is why.  Not because I support cancel culture, but because overgeneralizations and flase claims just exacerbate the issue.