It is fair to say, most businesses do what they choose to do. It is also true you must find some place to deal with your tank.
When the lady at Suburban Propane even quoted the regulation you're quoting and said there are some exclusions, did she say what they were? I mean no disrespect to the person who does this for a living, but it would not be the first time they were incorrect. Considering the cost of the horizontal cylinder, I would want to talk to the manager, and when I did it would be with a printed copy of the regulations in hand. If they simply refuse to fill the cylinder, you nor I can change that.
I went through this with the last TC I had. The real problem is when and if you get the problem solved locally, you may not be able to get them filled on the road.
Wayne