Registering with the Selective Service is not the same as the draft. Yes, any future draftees would be taken from those who registered, but assuming anything about what a future draft would look like is pure speculation. If Congress chooses to reenact the draft, they would design it in any way they see fit so it won't necessarily look like it did in the 70s. And using the draft as an argument in the drinking age discussion is silly because only males must register so using it as an argument then pretty much means that the drinking age should be lowered for men.

And having age restrictions only on the purchase, but not the consumption sounds reasonable to me.