I suspect integrators and users like to see specific models listed as being supported, even if it is the same underlying unit with a different label.
That's it in a nutshell. I can see the argument behind calling it "padding", but I don't think customers/partners are all that impressed with 6000+ supported devices. I think what matters to them is whether or not the handful of cameras they want/have are specifically going to be supported.
With so many OEMs, it's really difficult to know what the "root model" is of any particular device. And even if the original model is known, compatibility is not guaranteed as the firmware is not always close enough to be detected/compatible with existing drivers.
By explicitly listing each make/model that has been tested by Milestone and/or the manufacturer, partners and customers can be assured that it should work.