There is no need to stay "on the good side of the law." They pay off cops. They pay off the judges.

If they can kill with impunity they can certainly do the same with drugs.

Large-scale crack cocaine gangs existed too with substantial levels of organization but they still sell crack.

They're mostly unintelligent thugs motivated by greed. And I think the "Why would they kill their own customers?" implies they think that far ahead. Which is probably true as well for most major corps. They'll take short term profit over quite literally anything.