1. What the citizens want is ostensibly reflected by Congress--that's what being in a representative democracy means.
2. If some people have to register for the Selective Service, why shouldn't everyone have to? Better nobody, I agree, but better everybody than just some people.
3. Up here, at least, conscientious objectors were stripped of many civil rights during wars. They might still make you serve as a field medic, though. (And hey, there've been three pacifist Medal of Honor recipients, all medics:
Desmond Doss,
Thomas Bennett and
Joseph LaPointe.)