Why do I call transmitters controllers?

While the practice in the hobby is to call the bulky object you hold in your hands when controlling a craft a transmitter, this is confusing, as you can add a transmitter - a PCB board or module that emits radio waves - to a transmitter. Similarly, in the US (and probably most other jurisdictions) it's illegal to modify a device that transmits radio waves, as they are tested for compliance with the various regulations designed to prevent interference with other devices. So when you start talking about modifying a transmitter people legitimately worry about raising legal issues. The populace at large considers devices used to control things - the actions of a character or piece in a video game, a remote control toy, etc. - a controller. There's no danger of confusing the controller with it's internal transmitter. Likewise, modifying the controller of a transmitter doesn't raise the legal questions that modifying the transmitter does.