I think spawn trapping/killing does tend to ruin games IMO.
It means a team has learnt the map to an extent that they can manipulate the spawning of the enemy team who cannot change where they spawn once the trap is set. This is similar to 6 guys standing over people lying on the ground and kicking them in the head everytime they try to get up.
Is it illegal ? No.
Does it ruin games for people ? Yes.
Spawntrapping is that grey area where it takes skill and knowledge to setup and therefore is a valid tactic to an extent - should the team just say "ok we will sit back and let them possibly get an advantage" or should you press your current advantage and keep the enemy down and out ?
Either way it is an exploit of poor spawn logic coupled with good map knowledge.