Don't quote me on this Ack-Ack but the inability of buffs to fire while on the ground came about the same time as the nix on car bombing.
They are both measures taken by HTC to keep the "balance" between gamer's and simmer's (IMHO). Face it... in WW2 (before and since), massive numbers of aircrews don't, didn't, won't rush out to thier aircraft and attempt to get airborne (or just sit there) while thier base in under attack just to man thier guns. Take a look sometime, plenty of folks STILL up bombers in the middle of attacks... some actually get off the ground but I seldom see them accomplish much of anything, as it should be. Further... unless a RL base just happens to belong to a bunch of Al-Queda fanatics, the chances of hordes of idiots running out and exploding themselves and everything around them in the name of "base defense" aint gonna happen either.
I'm thankful for these things. I think most are... the whine-0-meter never really got out of the green when these measures were introduced.
P.S. There are times when bombers actually should be able to use thier guns while still on the ground. Heck, I've rolled a buff and been suprised by the lone cherry-picker before. Yep... that would be a good time to have the ability, unfortunately the "gamey hordes" ruined that.