I can claim I didn't possess the mens rea for theft. In order to be guilty of theft, I have to intend to deprive the store of the pack of gun without permission. If someone with apparent authority tells me I am authorized to take the gum if I sign a form saying I paid for it, than I haven't committed theft.
It depends on the situation. If the manager informs me the form is intended to trick the store owner, then I've committed a conspiracy. But if he's just like, "Oh this is for stock-keeping purposes," I'm not. His authority, just like the loan officer's, puts him on the hook, not me, unless I have reason to doubt the legitimacy of his authority.
And in the case of these loans, the banks had full knowledge and condoned the fraudulent loan applications anyways. You can't steal from someone who agrees to give you something, even if you have to lie in the process. You can't defraud someone who participates in the deception, or in this case, who puts you up to it.