If the endgame is simple, then they shouldn't lose on time if they have as much as 1 minute left.
If the endgame is not simple, and they're significantly behind on the clock, then they deserve to lose.
(If times are about equal then it's a bit dirty, yeah)
I just played a game which I was one pawn up in a rook endgame and it was a draw, my opponent had 1 minute left so I decided to go for the flag in which he lost his rook and got mated, is that un sportsman like?
Anything goes in chess! People who consider something like that "unsportsmanlike" are usually just frustrated being on the receiving end.