I respectfully don't agree with you. From a personal perspective, I sell items all year and have around a 3rd of the buyers use promo codes for discounts. We do not market the codes to people who would be buying and paying full price. That is cannibalizing profit. We only want people to use a code if they would not have purchased otherwise. So to say that the majority of codes benefit the seller is not relevant. You are stealing money from the seller if this code was supposed to be exclusively for a specific group of people and you are using the code. If an airline gives their sales team or travel agents a 90% off discount, you can use it because "the majority of times..."??
And circling back to the earlier claim - if they wanted, they can more strongly enforce the qualification of the code usage, I am at loss to understand how that makes it permitted to steal just because others do it and they left open the option by mistake Of course, sometimes they are happy with anyone using the code. The question is in a case where you think or know that they do not want you to use the code, just you think it is fine because they did not more strongly enforce it.
I'm not sure what business your running, but an average of a 3rd of your buyers using promo codes sounds like your benefiting from the use of those codes. And hopefully those are costumers which would have otherwise gone somewhere else, and are now coming to you.
Let's take the example you gave. An airline gives out a 90% off code and they make the code "90OFF", I would be extremely shocked if that code managed to stay just between their sales team. Obviously if your talking about a small grocery store that would be a different story.
As you yourself mentioned, it would depend what they were thinking when they made the codes. IMO no major established company makes a generic code without taking into account that it will probably go around the likes of SD etc..
If I may add, from the mere fact that it's a question, means it isn't clear if it's stealing (if it is please bring a source), it's quite easy to call people stealing but I don't believe it necessary.