Have you played the GC and 3DS Mario Tennis games? Power shots being part of the single player is nothing new.
It's not just Zone Shots though. It's about trying to force your opponent to waste their energy.
If you use a Zone Shot and then the opponent reaches it with little to no problem, then you've wasted energy and you're vulnerable to a counter attack.
Building up energy is not something that just happens over time. It's a reward for skillful play. Successfully pulling off Max Charge Shots, Early Trick Shots and Nice Serves all give you energy.
You can't just Whack out a Zone Shot and expect to come off on top. You pile on the pressure, make the opponent waste their energy and hit them when they don't have the energy to Zone Speed back.
I can understand being upset that it's basically a Fighting Game disguised as a tennis game. But the Tennis Game is still there if you want to play it. (Even if it's mostly multiplayer, but if you didn't get this for some Multiplayer matches, then I don't know what to say)