In the SLC500 the inputs have fixed system names (I:1/5 for example). But if, at the start of the program the whole group of inputs were copied to, let's say, a B register's word, you would have the same problem in trying to 'force' one of the 'B' bits. You would have to go back to the actual 'I' input bit to force it.
The problem lies in the understanding of the structure. Once you understand that 'Local ... etc' is the pre-assigned system name for the input and it was being copied to another memory register you would realise that you couldn't force that second register, you can only force the pre-assigned name.
But (to add to the confusion) the Controllogix family adds the idea of an 'alias' which is a whole seperate tag but can be DIRECTLY tied to the input bit. It is SO directly tied that you can 'force' the alias and it is exactly the same as forcing the 'Local' bit. This does not have a similar idea in the SLC500/Micrologix world. But we weren't dealing with the 'alias' in this case.