Hillary! uses this to her advantage by having thugs dress up like Secret Service for crowd control. The current President could change that in an instant by requiring an identifiable uniform. Impersonating a Secret Service Agent by wearing an identifiable uniform would most definitely be a crime. But any idiot can dress in a dark suit and sunglasses and stick a radio earpiece in his ear.
Another problem with the lack of identifiable uniform is that it allows people to get close to the President. You don't have to be Tom Clancy to figure out that dressing the part is the best way to hide in plain sight. Given our present situation, this is an unacceptable risk.