Futaba did not purposely make their RX's operate at lower than 3.3V - that's the natural progression of IC technology and everyone will be heading in that direction eventually. They could have added buffers to bring the signal levels back up so that they could operate other servos and associated equipment, but that would have increased the size, power consumption and cost of their RX's. They chose not to do that, and low and behold, most everyone else modified their stuff to be compatible - probably because they know that this is the way things are going...

Bob