They pull the authority from the preamble to the Constitution of the United States:
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
If you take the full preamble in context it isn't there, but that has never stopped people. This is the reasoning they have used to force businesses to serve those who refuse to pay - Hospitals can't turn away because of inability to pay, funeral homes can not hold up a funeral for lack of payment, (brain fart .... Just lost my train of thought

). Some have even argued and won with eminent domain to seize assets from business to procure needed goods for the community.
Show me where it is clear they can, and I will thank you for pulling the blinders off my face; show me where it says they can't, and I will agree with you; but accept that the interpretation has been perverted to mean what they want.