I guess the best way I can explain it is that you can also place walls (Linear Forms) and Slabs in Structural, you can even place generic PAZs. So those type of objects can be manipulated, at least in a basic manner. But there's nothing in Architecture that equates to what Structural copes do, and how the relationship between Structural members is maintained. IOW, Architecture has no means to move one member and have the other one auto-cope. Therefore it prevents you from manipulating them in order to retain that relationship.
Though not as extreme, it's similar to opening an Architecture file in plain MicroStation and attempting to move a wall.