Now, while I am far from being a defender of chavs, this "zero tolerance" policy, like all other such policies, is a ridiculous abdication of judgement on behalf of the security staff. This rule is aimed at excluding antisocial teenagers who intimidate shoppers, bully staff, and damage property. However, it will also affect chemotherapy patients who wear them to hide bald heads, American tourists who wear them as an entirely nonaggressive fashion statement, and people who are just wearing a hoodie because they get cold ears.
The security staff should be given discretion to decide who can and cannot enter the complex, and if their judgement cannot be trusted then we should find people who can, not place our faith in an excessively broad blanket policy.
I recommend Randy Cassingham's site on the damage being done by zero-tolerance policies in the US and elsewhere.