Often times (in the US) the 90 days notice doesn't mean anything badge/credentials wise. You are walked out that evening (sometimes supervised) and your credentials are revoked asap.

WARN layoffs in NY require 90 days protection.

https://en.wikipedia.org/wiki/Worker_Adjustment_and_Retraini...

Maybe to limit the damage notified (and thus possibly upset) employees can cause.

That must've been an interesting calculation. Now they're paying dozens of employees that literally aren't able to work three months worth of salary.

Severance is pretty common, and some companies will just keep you technically employed (and thus paid, and even given health insurance) with no expectation of you doing work or coming into the office. Three months is definitely on the generous side, but not that unusual from what I've seen and heard.

Yep. Severance is pretty common when employees are "laid off" i.e. their employment is terminated not because of employees fault. And generally, you are informed of it, and your credentials and such are revoked almost immediately.

Quite different from when you voluntarily leave, when you give notice and do knowledge transfer and such. I imagine that is because the employee is unlikely to do anything bad, since they likely have a better future.

They're going to place work second anyway (to finding a new job). It's usually to give them a severance, not screw folks on a visa, etc.

Spot on. You never know who might have taken the news in a bad way and decides to lash out.