Yeah, you probably shouldn't try to increase the speed...but it is fun to try. People have been modifying cars for years and there is a big market of aftermarket performance enhancing products out there....and very few are targeted at keeping or improving fuel economy.
I can only speculate why the speed is limited, but I can think of 2 possibilities.
1) The motor is likely brushless; which means a controller energizes coils in a sequence in order to make it turn. The faster the motor controller steps through the sequence, the faster it goes. There may be a limit to the rate at which the controller can step through the sequence.
2) The power consumed by the motor increases with the square of the speed. The batteries can only deliver so much power to the motor; however, I think this is less likely the case because the the maximum speed isn't reduced if you are going up hill with the heater on....at least I don't think so.
So if #1 is the reason, a whole new controller may be needed....or, put on bigger tires.