Signalling is optional in many states, if a bicyclist has to keep both hands on the bars for manuevering or braking....
despite what a bicyclist 'should' or shouldn't do, no road user has the right to enforce law by harrassing fellow road users.

the op ran into a jackstrap too stuck on himself, and likely not familiar with the signaling optional rule for bicyclists....i've gotten the reactionary "you're suppossed to be signalling" prior to some low grade motorist harassment similar to how you've described it.

too much shock talk radio into frustrated americans still stuck in their cars is take on it.