It's just one of those things. There are many players that play without calling the score, and who barely make a gesture or say anything when a shot is out.
I remember during one match where I hit a serve blatantly long, my opponent hit the ball casually, and didn't move, I got ready to serve again, and the guy says, "your serve was in."
He barely called anything, so it was pretty ridiculous. You just have to play every ball and beat these idiots into a bloody pulp. Coincidentally, these are the same people who don't signal when driving, and txt while in rush hour traffic.