I have definitely been that weirdo cutting foam boards to fit in my car in the Home Depot parking lot. Gotta do what you gotta do!
I work for the other store and cutting really depends on which employee you get. Some are sticklers for the 2 cuts rule and others can't be bothered to make sure you get charged. A friendly attitude does go a long way in getting someone to cut things for you too.
I've been lucky that everyone in my store knows that I'm into Halloween and love foam. There used be a guy in our assembly department that kept saving large pieces for me, but he left the company two years ago. I'm still working through what he gave me, but I'm going to run out soon. Also, everyone just assumes that when I buy something, it's for the haunt. The few times I've bought something and it wasn't for Halloween people asked if I was okay.