If I saw a cav player on my team ignore a free kill at enemy spawn I would be pissed. That enemy could turn around and kill me, a teammate, or the cav player later in the round.
Yes it's cheap, but it's battle, not an honour simulator. Same goes for kiting, backstabbing, you name it.
Also the only time this should legitimately happen is if you spawn in late to a game. If you spawn at round start and are still lagging long enough for an enemy cav to reach your spawn check your computer/internet connection into a nursing home.