If the Airforce was buying 7e7 tankers then they would be subsidizing boeing. Keeping open a finished production line is hardly subsidizing passenger aircraft developement.
Except look at the MD-11: If the KC-10 order had not kept the DC-10 production line going, there never would have been an MD-11, as the gap between the last orders for civilian DC-10s and the first MD-11 was far too big.
Similarly, keeping the 767 line open helps Boeing by (i) keeping buildings and employees going until such time as the 7E7 reaches production and (ii) provides a steady flow of income that can help finance R&D for the 7E7.
It's not exactly the same as a direct subsidy to 7E7 development, but it'll do.