If you are out numbered then yes it's poor sportsmanship to sneak on you, however if the teams are even, or close to even, then it's up to the pilots to pay attention.
If you don't see a player after awhile, check the roster to see if they are still in the game. If they are, they may be parked, or they may be sneaking. It's at that time you use team chat to alert your teammates of a potential sneaker. Then you defend against him.
If your teams are even, or near even, but someone keeps sneaking and getting your hangers, you only have yourself to blame, not the act itself. Become a better pilot and work on team work, rather than asking it to be illegal.
With that said, once again it's not good sportsmanship to do it if you outnumber the other team, however, you will find some pilots who don't care, they just want to win, and get hangers.