It's supposed to be just a gentle bump to get your attention, not a full-on ramming. Sometimes the dust is so bad that you can't see them coming up behind you, particularly if they're a fast vehicle. And sometimes they're into your rear bumper harder than intended because they couldn't see you either.
And anyone who even lightly taps a Class 11 (Full Bodied VW Bug) gets shunned by everyone else. Legal or not, there some things that you just do not do, and that is one of them. No matter what, you do not do that.