Um...pretty sure the same argument can be made about illegal immigrants just minding their own business and working under the table
First, Working under the table doesn't stimulate the US economy,
and second, if the money is going back to mexico (which it usually is) then it sure as hell isn't helping the US economy.
This is why legalization of certain drugs, along with the fact that we'd have more control over the substances, would greatly help the US economy