If you were not a good enough player to checkmat your opponent during the game, don't begrudge them for practicing their end game skills on you. For some people it is not just about winning or losing, it is about learning.
Thirded.
Also, I recently had some games (both won and lost) where in the win the other player was lost and even said good game before he made his last move to let me checkmate him. Afterwards he said he did not resign because it was a hard fought battle and he wanted to give me the satisfaction of the checkmate. A few games later I was in the same boat on the losing end and I did the same for my opponent. Sometimes I will resign and others I will let them checkmate me for the prize.
After all, it is just a game.
My advice? Get serious about chess or stop playing competitivly
but most of them are bullet 
If you were not a good enough player to checkmat your opponent during the game, don't begrudge them for practicing their end game skills on you. For some people it is not just about winning or losing, it is about learning.
Fourthed.