As I was typing my post, GUWonder's post showed up and answered most of my questions about the commercial motivation here.

It does seem obvious that fire codes are not the issue here -- something like min. sq. m per occupant.

Some of the Hyatt suites that are limited to 2-3 occupants are in fact 70+ sq m, which is larger than some family apartments and presumably does not represent a dangerous level of overcrowding.

There is also a lot of inconsistency -- the Hyatt Paris Madeleine explicitly allows 4 people in a room that is only 28-36 sq m, whereas its friend across town, the PH Vendome, does not allow 4 people in a deluxe suite that is significantly larger (50-55 sq m).