Because you are better than they are?

edit: Since I can't delete my comment now, I just want to apologize that I read your message too quickly and missed the 50% salary difference part. Come on, there's ethical and then there is crazy, of course you leave and take the other offer if it's 50%, that's common sense and any hiring manager should understand it. But that's not really commonplace and you can handle it in a professionally ethical way.

If a relationship is bound by ethical principles on one side and a cold economic calculus on the other, it seems pretty clear which will be taken advantage of most often.

I don't see anything particularly wrong with either way, I guess. If they have to rescind offers to be a profitable business, so be it. Sometimes I will have to renege in exchange for working fewer years.

I'm joining a business, not the Knights Templar.

... And how do I put 'better than they are' into my 401k?

Listen, man. I am a hiring manager. People have reneged on accepted offers. It doesn't affect me personally at all. It's a bit annoying, but the real damage to real human beings is close to zero. OTOH, a company rescinding an offer to someone who accepted and probably quit their other job can be devastating. It happened to fried of mine very recently and their life is very stressful right now. You are definitely arguing on the wrong side of this.

Why? I am not dealing with a person. The ethical reasons for being honest don't extend to groups of people acting like psychopaths.

Don't let profit driven corporations brand themselves as "family", or any other concept which deserves moral consideration.