Why do tennis players apologize for a net point won?

Answer 1

Tennis players do not apologize for just any net point.

Tennis players usually apologize for the cheap shots, like if the player hit a forehand, but it hit the net at the top and bounced right in front of the net while the opponent was at the baseline.

Hitting the net is never the goal, and is an accident, especially if it goes over. When it hits the net and goes over, that is when players usually apologize for the cheap win on the point.

On regular net points, players do not apologize to one another because it was won fairly without any cheap play involved.