I don't know, as long as lawyers are allowed to defend murderers, you cannot really argue why they wouldn't be allowed to defend companies in such a case.

There is a big difference between defending a basic human right (e.g. a murderers right to be judged fairly) and a company explicitly putting illegal clauses in a contract to bully their current and past employees. An ethical lawyer would (and should) pick up the first case, and advise the company to remove that clause in the second case, because its illegal and not enforceable, with its only purpose is to bully people.

There is a difference between defending someone who is indicted (the lawsuit is going to happen no matter what, and the defendant has a right to a lawyer), and initiating a lawsuit.