Because the transponder is powered by electricity. Electricity is supplied by wiring and that wiring needs to be protected from short circuits or overload otherwise the wiring will get hot. That protection is in the form of a fuse or circuit breaker. This applies to all wiring. To turn off the transponder you just need to work out which circuit it's on and switch off the circuit breaker. It may be in the cockpit or outside the cockpit. Someone here will know but may not say.
