There are actually some cases when members got killed or shelved for dealing. But they are not many. And these members were not always caught by the cops, but by the boss. Disobeying the boss´s rules show a lack of respect for him, which is certainly a big nono. But I agree with you. Only a very small fraction of those who got caught dealing suffered the consequences. Probably this has to do with how wide spread the problem was and how serious the consequences could get. If knowing half of his Family is dealing, killing drug dealing soldiers and captains could backfire on the boss. It also has to do with nepotism of course. But if the boss didn't like a certain member who was dealing, and particularly if that member wasn't generally liked by the other members, he could have found himself in deep trouble.

And yeah you´re right. A boss will not kill a member of his own Family just because another boss demands it.