A question for authors

There is something I've always wondered about. Some authors seem to refuse to real world references. They write a story set in the US, but the characters shop at stores called Q*Mart, drink Fake Cola, eat at Tennessee Fried Chicken, and so on. Other authors seem to have include a TM or Copyright sign after every trademarked or copyrighted name. I don't see this in mass market fiction. Are those authors afraid of being sued? If I'm Coca Cola I'm suing someone with money, not some unknown writer on the internet.

Can someone that does that, or did that explain why?