It's not really a flaw in the game though, you just kind of state that like it's a fact. It's more of an idiosyncrasy of the game of tennis. If you could hit a sharply angled slice serve from the deuce court in real life, any time you wanted, it would be just as effective. I have played guys in real life where I am thinking "You cheesy bastard stop hitting that serve"

Because it really is just as difficult to deal with. I guess the difference is that in TS3 and most tennis games, this serve is very easy to hit with great consistency. But nobody ever said TS3 is real-life tennis or should be
exactly like it, nor should we impose physics/expectations of real-life tennis onto TS3 or any other game. The similarity to real-life tennis ends with the fact that you are hitting a (virtual) ball back and forth over a tennis net. Everything else is up to the game engine/physics and you should judge shots/strategy in TS3 with that firmly in mind. Can't blame anybody for taking advantage of something in a game. To not take advantage of some extremely efficient techniques is foolish.
Don't hate the player, hate the game.