I believe that this remained the habit of motorcyclists until relatively recently and it has since been tested in court and declared not allow (reference vague memories of newspaper articles).
Do motorcyclists signal to others except when there is a speed trap, or do they signal to others when there is a speed trap? The later is common in the states (cars flashing their headlights), but I haven't heard of the former and enforcing a ban on it seems completely impractical at the very least.
And what kind of free society can force citizens to salute? "The government won't tolerate warrant canaries" makes intuitive sense because we have grown used to the courts throwing out all sensibility whenever there are computers involved, but the idea of the government compelling civilians to salute "in meatspace" seems blatantly beyond the pale.
We tap our helmets to warn of cops. We wave (or nod in countries that drive on the left) just to say hi. Absence of a signal would be useless on a bike as we're generally pretty bad about giving the signals we intend to give, let alone not giving the ones we don't intend to. It would be chaos.
