Neither has priority. That said, your letter to the business was pretty well stated.
I would agree that in the letter of the rule, neither has priority. However... In practical use, I've had a license for about 25 years, and there may be 3 other legit guys within 25 miles of me with a license as well. With that said, channel usage is not a problem. Owning an expensive repeater, I simply don't care if some FRS-er is jacking around on their $22 radio, or a business just happens to start using 13 new radios they got at Menards or Walmart and think that gives them exclusive use on a certain channel. I use my repeater when I want to and they aren't going to get in my way. If I hear a licensed user, I'll give them room.
There was a medical office on the same freq that I used for a secondary open repeater once, and I advised them they could not use GMRS (this was 10 years ago). They told me to "get off our headsets". No problem, I made the repeater ID every 15 minutes on the PL they were using. Didn't take long, they were gone. My theory is, file a complaint, we'll see who wins.
It is amazing how much of an expert some people are on some things, they really are, but some of the same people pick up a radio....heaven forbid a UV5-R, and think they can just do anything they want. I make the analogy that you can buy a Mustang that goes 150 Mph...but you aren't allowed to do that on the Highway.