My guess (and it's simply that and in no way the official Sprint answer) is that it was to discourage employees or friends/families on employee accounts from buying the phone off contract in the first place. If an employee account obtains an iPhone and adds it to the employee account, Sprint doesn't really get anything out of it since they are either $0 or heavily discount rates to begin with. By implementing this "policy" it's more likely that the phones will be sold to "regular" paying customers.
Again, I don't know if that's the official story and honestly it was never a big deal for me.
I can see that, I suppose. But they eventually lift (Verizon) this restriction, don't they?