People with mental or physical disabilities... [snip]
People with mental disabilities are exempt from minimum wage laws. You can hire a mentally retarded individual for $1 per hour if you want.
They are exempt because if you require an employer to pay $15/hr for the labor of someone who is mentally retarded, that employer will simply not hire those people. Those exemptions are put into the law for this very reason! It is much better to have these unfortunate people hired by someone, staying active in something, instead of just sitting at home.
The same applies to interns. You can hire an intern for $0/hr. Is that exploitation? Many interns would disagree.
If the value of your labor is less than the minimum wage, then you are unemployed.