If there are differences between models and differences between the real thing, has anyone considered trademark or copyright laws as being the reason? 

Nope. It's held that making a model be a plastic one or 3d - of current miliatry hardware is 'OK' because taxpayers have paid for it. Thats the approach plastic model companys have used sucessfully to argue over c/right and licencing issues. *Source: Finescale Modeller. 
*Good points there Joe - though 1 thing collectors forget is in the real world army stuff gets modifed all the time so very often theres no single defination of whats or wrong.

While painting a bailey bridge 2 crews started at each end to see who could finish first. 
Finished and having a brewup we noticed there where two very distinsticve shades of olive green paint! 

Pinky - you left the lens cap of your mind on again.