There are some problems with allowing that though. In some cases a clueless CSR could allow the person paying for the domain to gain access to it. The assumption is wrongly made that if the person is paying for the domain they have rights to it. After paying they will simply say something like "oh, by the way my address needs to be changed" or open a conversation about something else and end up gaining access.
Someone once renewed a hotmail domain after Microsoft forgot.
Registrars should allowing gifting, source doesn't matter.