Possibly because experience suggests those wearing crash helmets might be robbers, and those wearing hoodies cause trouble?
When did you last hear of a Muslim woman wearing a bhurka robbing a petrol station or chavving around in a shopping centre?
Also this is a point of Law. You can wear what you like in this country, it is a free country, no law tells you what you can and cannot wear in public. But privately owned places like shopping centres, petrol stations and nightclubs etc can discriminate on what you wear, whether or not they want your business based on what you're wearing is up to them.