Even when they're caught, they get put on a Performance Improvement Plan (PIP) which is shorthand for giving them 60 days to find a new job or to turn their life around internally. Usually, it leads to the former, sometimes the latter. I've seen both.
It's an imperfect and biased process. But the attempt is usually made because HR fears unjustified termination lawsuits despite the "at will" employee status of just about everyone.
Unless they've tried to hack the company's servers for private data, I've never seen anyone fired on the spot without the above process unfolding. Maybe your experience differs?
PS I also think Charles Manson and The Unabomber were unambiguously guilty. That doesn't change my opinion that they deserved the trial they got.
PPS If as amyjess seemingly suggests that female professors at MIT repeatedly filed complaints against him and nothing happened, well then carry on Twitter mob, good job, seriously.