A literal bus factory may not be critical for national security, but the ability to manufacture a vehicle is. So the know-how, the supply lines, and the manufacturing facility are important. The ability to manufacture a fuel injector, a transmission, a windshield is going going to apply to a bus, a plane, a tank..
So subsidise the bus manufacturers to make competitive products directly, rather than an indirect subsidy via forcing transport authorities to buy uncompetitive junk.
Forcing transport authorities etc to buy local seems like clearly the worst way to subsidise industry; there is little incentive for the manufacturers to make a good or cost-competitive product.
Sure that's why the Hummer was a great vehicle with all the institutional knowledge from GM. /s Also modern engines in tanks and planes are turbine engines with nothing in common to lighter vehicles (APCs trucks etc). Tanks don't have windshields either.
