The implication of ownership that accompanies slavery. Conscription may be considered involuntary servitude, but the 2nd Amendment effectively makes every adult a member of a state militia. The 'involuntary' aspect is negated by the fact that one is not involuntarily a citizen. You may argue that a citizen has to express their right to bear arms in order to fall under the militia status, but it's generally accepted by lawmakers that the right to bear arms carries the stipulation that the government also has a right to call upon the citizens in times of dire need. This is, of course, specifically for America, and it's my humble opinion that any president that institutes a draft would never hold office again and it would potentially fragment his or her party permanently.