In a wonderful world - the api wouldn’t do it. It causes problems in the ui world since you could end up (and will end up) with:
Each one can have their own list of users so each one is valid.
So why is this bad? Well if you’re saving the group name in your settings somewhere - and then the admin changes the casing of the group - that should cause any problem right (because Dwester is the same as dwester)? Queue the support tickets…
So why does the api allow it? External evil directories that allows it (but they have more metadata about the groups) and we’re left with cleaning up after them.
Not that I have any opinions about it.