If an employee has been malicious during their tenure then other systems should have detected and caught that. That scenario can be decoupled from the termination scenario.
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.
And for some reason there's a particular country in this world where this is interpreted in one way and one way only (well not 100% true either but the odds of being treated one way are waaaaay skewed towards what you're saying) while in other countries it's interpreted in other ways and they're all just fine. I've personally witnessed and been part of the other squares over my entire career.
If entire countries can run on the other squares, why can't the US?
Some countries selectively put some people in the "you have a very long notice period" and "you will not be working for us, but still be paid, during your notice period" (so-called gardening leave).
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.
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.