"Aye, sir!"
It's not the navy. You're not under orders. They can fire you if they want. But if that's not enough for you: promises and loyalty works boths ways. Suppose you're a veteran at twitter. The company (you thought) joined is not what it is today. You're not bound to any original promises.
Furthermore, you didn't make any promises. You signed a contract, which is only partly binding (check your jurisdiction for details; e.g., where I live, a non-compete clause is frequently overruled by a judge). You didn't promise to increase shareholder value on your mother's life.
Finally, when the interests are substantial enough, ethics trump loyalty and honor. I don't think I have to substantiate that.
Of course, this doesn't hold when you adhere to some kind of "corporatism", where the state and/or corporations decide what's good for you.