Some zoos will castrate some males for various reasons... health, age, too much of that bloodline, etc, etc, etc. But that goes for many species, not just zebra. As to your question zebra are born in zoos all the time and they will continue to be bred as it is a easier and better option than removing any from the wild.
My guess is most zoos do not sterilize zebras and there have been a few zebra live births at zoos.
Or do they sterilize zebras in the zoos?
