One key fob per parking space is pretty much the norm. Imagine everyone had a "spare" - you would never find a visitor parking spot... and those that own a spot may find a car parked in their spot... as with the $10 spare key - the cost is pretty norm too... however, you could have just bought one from a lock smith and get them to do it at the same time... it would be cheaper.
Are you breaking any laws? no... is the key special? probably not...
What I am surprised is that they wouldn't give an extra fob... it's a real inconvenience if you are a landlord.