There has been a long history of Boeing and the Department of Defense attempting to cut a deal to avoid competition on this program. In fact, a DOD official went to jail over this.

They have now jointly come out with a statement that the tanker order should not be split, claiming that the logistics and development duplication would be prohibitive.

This is a lie. There are numerous airlines that operate both the B-767 and the A-330.

As to development costs, both manufacturers are making flying these tankers in preparation of delivery overseas customers.

What is true is that it would completely politically untenable to shut Boeing out of this deal, and by adding this sort of requirement, you make Boeing the only choice.