Laws forcing patronage of a business
I was talking with someone the other day about how I had to take a detour on my way home from work due to road construction, and the new route passed by a Krispy Kreme. I joked that "It is well known that if you pass within 500 feet of a Krispy Kreme you are required by law to purchase a dozen donuts. So being the law-abiding citizen I am, I did." Obviously this is false; such a law would surely be illegal.
But... exactly how is it illegal? What part of the constitution or federal laws says this is no bueno? (I imagine a similar law requiring everyone buy, oh, McDonald's for lunch at least once a month would be illegal for the same reason, but if the change in what forces the patronage actually changes what makes it illegal, do let me know!)