Not that I agree with this, but, the federal government does this sort of linkage of issues in other areas.
States were compelled to raise their drinking age to 21, in order not to lose federal highway funding.
States and localities are compelled to comply with many regulations in education, otherwise lose federal aid to education. The school lunch program is another area in which they are compelled to follow the standards otherwise lose federal funding.
Years ago, I worked in a college business office, and we administered financial aid. A decree came down that male students had to provide evidence of having registered for the draft, in order to receive their student loans. Not sure if that is still enforced, but, is another example of the power of the federal government forcing compliance with their edicts.
Yeah, they have "threat commercials" now on talk radio about how they "gonna eff up you life" if you don't register.
Fortunately for Obama, he was a foreign student, and didn't have to bother with signing up for the draft - until he had to pretend he wasn't an Indonesian Muslim, but an American Christian.
They had to scramble, but the Donks knew a guy...