How do local farmers markets affect interstate commerce?
There's a case - a famous one, though since I am not a lawyer I don't remember the details - where a farmer in Ohio I believe grew wheat for his own use. Again I don't remember the details but:
Long story short the SCOTUS ruled he was involved in interstate commerce even though the wheat never left his farm.
Some lawyer type will jump in with the details of that case shortly I imagine.
From my point of view the answer to your question is simple: because the feds say it does. That's incredibly wrong headed but it's pretty much the way applying the interstate commerce clause works these days.