Well, for one thing, many players go into Ground Forces because they want to play tanks. Having aircraft already redefines what that means. If you had SPAA in air battles (so when everyone's in an airplane, just to be clear), and those SPAA had laser-guided missile launchers that you had no chance to avoid, I think there'd be an equivalent amount of disgust and loathing from the air players.

There's no real way to balance aircraft in a tank game if you give them the ability to bomb/rocket people to death the moment they're given free reign to do whatever they want; without a dedicated SPAA and people making sure there are at least two fighters on every bomber, it's very easy for a strong push to be denied by some chucklehead with a B-17 (bombing 4-6 people in a row with no repercussions). It's not so much a "whine" as it is a massive hindrance to gameplay in Ground Forces that causes skilled play to be thrown to the wind; essentially whichever team can rack up enough AI kills can start whiping out the biggest threats early, thereby securing an advantage with practically no thought involved.

To elucidate upon the point of it punishing skilled play, the moment you become a threat to the enemy team, taking over 5 kills, is the moment they're going to be gunning for you every time. At that point, you HAVE to be the going into the fighters to just be able to protect yourself from the bomber killstreak spam, which negates your contribution to the actual important battle (you know, the actual ground forces).

Which reminds me, the killstreak bomber spam is insane, and essentially makes a winning team extremely difficult to stop. It makes a snowball effect in the game that makes even matches a rarity and promotes what is essentially a form of griefing, with bombers taking out ground forces at their spawn, when they're finally in a good position, when they're flanking, etc. It negates the ability of ground forces to act, while promoting poor flying skills (read: suicides every time), thereby making ground forces overall a poorer experience.