Could also just be because someone can't afford the real game yet.
But you're right. There's really no reason to play a demo that many times, but the arbitrary play restriction seems absurd. Nobody else does this, and the Wii didn't even do it with the WiiWare demos.
I have personally only played most demos 3-4 times at the most. And 98% of the time, I only play the demo once and never touch it again.
The only exception for me was Denpa, and that was because the save transferred, so I kept playing like it was a real game.