Frankly I find it just a matter of basic human decency and morality to point out a missing item on a restaurant check or any other situation where based on somebody else's mistake you could gain monetary advantages.

Because it's wrong.

Call me a blind optimist if you will, but I furthermore very much doubt your numbers and believe there are quite a lot more people in the world that would not steal given the opportunity.
I'm sure there are countless of papers on this topic.
Regarding all of humanity to be similar to oneself in morality is dangerous. (whichever way it goes).

One thing does not have to do with the other. We all agree that LH should reimburse you and it not doing so is wrong. We can very much criticise you and still agree with you on the fact that LH is in the wrong in this case.

Looking at the bigger picture one could argue that the purpose (LH survival and eventual refund of all the money) justified the means (in some cases illegally not doing all they can to process refunds) but that is a whole other discussion.

That does not give you any justification for self justice, as you regard keeping a double refund.
Not regarding your actions, or more the lack thereof, as at least morally wrong is unsettling in my humble opinion.

Furthermore breaking the speed limit vs. intentionally keeping money that doesn't belong to you is hardly comparable. (legal ethics 101)

We're not here to belittle or lecture you or your ethics, however, I hope that you take the several comments you got regarding said action as constructive criticism that might lead you to reconsider your stance.

On that we agree.