Your opponent most certainly knows their own deck better than you do. Perhaps they know that the action taken will cripple them to the point where continuing to play would simply be drawing out the inevitable, a waste of time. I often concede when I know there is no way that I can win. At that point, it's no longer really a game. IMO, the sensible thing is to waste no more time, concede, and move on to a new game.
There are also people who just dislike playing against certain strategies. It's simply not fun for them. One of the objectives of the game is to have fun, so why keep playing if you're not having fun?
I have no problem with anyone conceding, for any reason they want.