Because one shop would have to spend bucketloads of money complying with anti-money laundering regulations. Bitcoin ATMs solve the problem in a much more economical way.
What makes you think that FinCEN is going to consider Bitcoin ATMs to be exempt from AML? Believe me, they won't. That's why it's been so hard to get them going.
But I agree that cash businesses also must comply with AML if they want to stay out of jail, BUT cash businesses don't have to put up with banks cutting them off at every turn. And if we develop our own clearing houses....
I believe there are bitcoin ATM machines planned for at least two cities in America soon. I might be wrong.