I don't care about these theories because there's nothing much I can do about them. However, I see plenty of so-called "skeptics" that don't question anything and they're always ready to just swallow the official story without even putting much thought into it.

Anyway - who said it has to be "all airlines"? Does it have to be 1 vs all? Couldn't it just be some percentage?

In Operation Mockingbird [1], the CIA found that infiltrating just 25% of media outlets was enough to sway public opinion. That's the type of operation that is obviously still ongoing too.