That's because the TX stick is made to work for airplanes. Note that its range of motion is a square. You can move the ailerons full blast and that's entirely separate from the elevator, which can simultaneously be moved full blast. But on a heli we don't have two separate gadgets connected to that stick, we have only one thing, the swash. It can tilt just so far in any direction. Its range of motion is bounded by a circle rather than a square. If you have the endpoints set to max for aileron only and elevator only, then you have a situation like this... the circular limit of the swash is INSIDE of the square of the TX control. This means that when you move the TX stick into a corner you're outside the range of the circle and binding happens. The "cyclic ring" keeps the TX stick within the circle. Alternatively, you can reduce the aileron/elevator to about 70% of max, which put the square of the TX stick INSIDE of the circular limit of the swash. In that case the corners don't exceed the limit of the swash, but you don't have max control otherwise. That's no big loss unless you want all you can get out of it.