It seems like poor sportsmanship, but it's part of the game, and adds a new dimension that can take you to the next level.
Situation: Your opponent has just played a full lock down safety, and you have no idea how to make a legal hit on your ball. In fact you may even risk hitting in the 8 ball. So most people would pick up the cue ball and hand it to their opponent. Wait! If you are going to give him ball in hand, why not degrade his layout on the table first. My favorite is pushing my opponents ball to the rail near the side pocket, or a pocket that I have blocked, or hit away a setup ball, or simply hit one of his ducks to the other end of the table. There are all kinds of things you can do when you can hit any of their balls, and it's kind of fun! Hopefully they understand how brilliant your shot was, but this is a legitimate way to play defense in 8 ball pool, or even 9 ball. Give it a try. I promise you can turn the table around in your favor in a hurry. Your opponent may not be happy, but you've just demonstrated that you are a master of the game!