I don't like it, but I don't think of it as being patronizing or done out of lack of respect. Lack of understanding I agree with, and you and LBM touched on the reason.
I don't like it because, while the motorist doing this may simply be trying to be nice, if another motorist is present they may not understand why the first motorist is doing what they are doing and blow right on through.
I don't like it because someone making an exception to the rules of the road, even if the intent is to be gracious, is not acting in a predictable manner, and can potentially put others in a dangerous situation.