Because Yes, he can

A bill introduced in the House of Representatives in the US forces all citizens aged 18-42 to serve for 2 years of mandatory service in whatever capacity deemed necessary by the US president. This may include mandatory military service in times of war (or military service in lieu of civilian service at the option of person), or civilian service at the discretion of the US President. That's right, the President can name anything he deems to be a worthy cause and force people to serve in it.

Read the bill here.

Now, I'm not going get very hysterical about a bill like this - I think that it's good for some people to give back to the community. I don't agree that it should be mandatory service. What I find funny is that someone who would introduce this bill obviously didn't think it through. Let's assumed that this bill was passed in the wake of 9/11. What are the odds that the President at the time would use a bill like this to force participation in that war? How much opposition would there have been at the time? What do you suppose the average American's reaction to the use of such a legislation would have been?

Long story short, this bill will do a lot of damage from unintended consequences. Hopefully both Houses will ensure that it never gets passed.