I've actually never once in 30+ years been asked to remove my helmet at a garage and do leave it on 99% of the time.

I always take my sunglasses off though so that people can see my eyes.

I must look the most unlikely potential thief ever??

It wouldn't really bother me if someone asked nicely but if they were rude I would become hard of hearing.

Personally if I was working in a fuel station I'd be more scared of a Burka wearer than someone wearing a bike helmet.