Probably, to prevent abuse. If someone is temp banned for a week or so, and this is public knowledge it could come up later down the line. I'm not sure about displaying banned members. It can be useful for reporting, but I guess removing their message if it infringes the guidelines is probably the right thing to do anyway.
I have always wondered, why we can't see who is banned and who is not. To prevent abuse which you mentioned, temp bans shouldn't be shown, and perm bans should.
I think, best case scenario everybody should see who is banned and who is not. If that's not an option, users who actively reporting should get this privilege, as a small bonus.