Even the "90-day probationary" periods are not really useful. I think the only thing that they do, is if the employee quits before the 90 days are up, then they have to pay the company back for all the expenses incurred by the company (I had this happen to someone we hired. They were not expecting that. Too bad. They were actually very good, and dumped us for a job in a location they preferred. I felt bad about that. I actually didn't hold any rancor towards them).

I suspect startups can be a lot more likely to be able to give someone the boot in an efficacious manner.