I don't think he said that. He says that your goal may be to save the world by being a public defender, but when you start working as a corporate lawyer with a nice apartment, you stop caring about the goal.
What he's implying is that it's "bad" to throw away your life-long ambitions for a bit of extra money. If your life-long ambition is to be a kick-ass corporate lawyer or investment banker, then there's nothing bad about that at all.
My only friend who became a lawyer who didn't "sell out" is the one who never planned to become a lawyer. He was an electrical engineer who was hired as a technical expert at a big IP law firm. They eventually offered to pay for him to go to law school. Everyone else I know wanted to save the world and now works for The Man. What the blog author writes is very true in that regard.
What he's implying is that it's "bad" to throw away your life-long ambitions for a bit of extra money. If your life-long ambition is to be a kick-ass corporate lawyer or investment banker, then there's nothing bad about that at all.