Which is still bonkers. Plenty of people with high level access have notice periods of 6 months in some European countries and work their remaining time after giving notice or at least part of it, because they negotiated the 6 down to 2 or 3.
Think about making a 2x2 matrix (nod to Pascal’s wager):
Across the top: fire immediately, don’t fire immediately.
Down the side: employee is benign, employee is malicious.
There’s only one square where the company is at risk- bottom right.
The algebra is easy: terminate immediately and the company is not at risk.
If entire countries can run on the other squares, why can't the US?
Not unusual in the finance industry, somewhat unusual (but I have heard of it) in "more pure tech". Probably also more common the further up you get in the corporate hierarchy.