Some vets won't do the deed til the chap/gal is at least 5/6 mths but some do it early-as for ethically correct well yes i think so as it's the only sure fire way to ensure your kitt isn't gonna be bred off or used for stud work which can cause a whole loada problems. We don't neuter/spay our kitts before they go and thus far i guess we've been extremely lucky in that all our adoptive families have honored our requests
For us this may well change in the future, i dunno about enforcing it as it were but being there kitten to start with they can refuse you the kitten full stop-i suppose it depends on the breeders
I'm sure if you already had kitten then they may withhold any papers etc