My local garage started doing this after suffering with lots of drive off's without paying and also after being robbed and one of their cashiers being seriously injured by someone wearing a crash helmet. They also won't activate the pumps at night without paying first, and that applies to cars as well.
They know me so i don't have to, but to be honest it wouldn't bother me taking my helmet off, its no big deal and i always take it off at petrol stations and shops that don't me, rather than make the cashier feel intimidated or at danger when i go in to pay.
Two sides to every story. I can see why some petrol stations feel the need to do this and respect their wishes and right to do so, or if i don't agree with something, just go elsewhere.
Reading the letter from Shell, them saying about asking for proof of age at the desk, for certain customers, doesn't say this will done after the pump has been activated ??