Sadly it may be true that because the corporations deal with such huge amounts of income and probably have to deal with going to court a lot because of lawsuits or what not that their lawyers would more than likely skim off the top whatever they can because there so much of it going around.
You need to look at the facts. Take this as an example:
"I am not sure about ‘all’ lawyers. But I have not, in all my life, found a single human being, whether lawyer, doctor, priest, bricklayer, judge, who was not, at some point in his life ‘morally corrupt’. A minor example: If a patient is dying, the doctor says to him, ‘take your medicine, go on. It will make you better’. Is that morally corrupt? A group of aggressive and angry men pursue a man, intent on killing a man. The victim rushes into your house. And pleads for his life. You hide him. And send the aggressors the other way. You lie. And you find that the man is a priest at a local parish. Are you morally corrupt because you lied? Change the scenario. The victim is a wanted paedophile. The pursuers were parents of a child. What then? Are you morally corrupt?" - GK Ganesan
Everyone can be corrupt. As lawyers they have to take time and learn about their cases.
"Sometimes we are required to keep things to ourselves that we don’t necessarily want to keep to ourselves. Sometimes we have to defend people that we may not necessarily want to defend, for all sorts of reasons. Sometimes we find out things about our clients after we have taken on their cases that we don’t like. But we, like you, have a job to do, and an obligation to do it well." - Ellis J.
Lawyers do things they don't want to do but they took an oath. Not all lawyers are bad, people just make it out that way because they might be upset about how a case turned out and much much more. In the end, you can't call them corrupt, just LOOK AT THE FACTS.
The idea that most corporate lawyers are corrupt presumable stems from those with the opinion that corporate lawyers are allowing corporations to get away with whatever maximizes their profits the most without any consideration for those who are harmed. Just because this practice is despicable and unpleasant does not mean that corporate lawyers are corrupt (unless you mean morally).