My understanding is that at least parts of the contract would be in annual reports and otherwise available to shareholders as some sort of required disclosure since the details of any such contract, including working conditions and employee benefits as well as salary increases, would be expected to materially impact stock prices. Moreover, when it's negotiation time, one union always seems to know details of the contract that was just negotiated by a different union or with a different company in the same industry for benchmark comparison purposes. [This wouldn't apply to privately held companies or airlines that are government owned, but other regulations could compell disclosure in these (rather rare in the context of this discussion) cases.]