The rationalization aspect of their model can't be overstated enough in my opinion. It never starts with something clearly unethical; it starts with something more complicated. Something that is uncomfortable and morally suboptimal, but has some justification being appealed to — it benefits the group, it's otherwise unfair for some members to have to bear some small temporary cost for the benefits of others, or something of that sort. The level of the corrupt behavior becomes more and more extreme though, such that justification becomes more and more questionable, until you're left with something more seriously problematic. In the meantime, the people who questioned the slippery slope might have left, and you're left with people who aren't in a position of power for whatever reason (they're junior, or in small numbers) to question what has become clearly unethical cultural norms.