Not only are they legal for cyclists and motorcyclists, but you made your intentions clear, which they admit by saying you are using the "wrong" arm signal.

Don't these people realize the only reason you can't use a straight right arm in a car is because the drivers arms aren't long enough to fit out the right side window! I've gotten yelled at for this before and I'm thinking of printing up little copies of this particular vehicle code to carry with me on my commutes.