That's mostly true, but I think there's still a bit of nuance; while there isn't really a defined list of acceptable reasons, there are some reasons that are specifically not allowed, like firing someone for race, gender, etc.

Of course, you could argue that being allowed to fire people for pretty much anything other than discrimination makes it an uphill battle to prove it if you were fired illegally, but I'm not attempting to make a defense of the system, just explain my understanding of it.