I will readily admit I'm not an expert. But, I did once work at a hotel that was being renovated. When the fire inspector came to review the changes, two items had to be verified.

First, the layout of the room was examined. In our case, nothing was really changed. The furniture (bed, desk, chair) were still in the same position and the size remained the same. If we had moved things, the inspector would have needed to make sure that the open space still allowed guests to evacuate quickly.

Second, the total maximum occupancy for the entire floor had to be determined. During an evacuation, it's not enough just to get everyone out of their guest room. Guests then need to go through the hallway to reach the stairwells. Since we hadn't changed the room types, that wasn't a problem.

However, if we decided to move all "double" room (max occupancy 4) onto the same floor(s), we might have problems because that could lead to a dangerously overcrowded hallway in the event of an evacuation at 100% occupancy.

-----
All that said, I'd guess that troubles you are experiencing are more related to lackluster individual hotel management than fire codes or company-wide standards. All of the major hotel groups (Marriott, Hilton, etc) tend to be hands-off when it comes to letting their hotels run their operations the way they want. If hotels set their maximum occupancy for a room with two beds to 3 people, Marriott isn't going to challenge that action.