I am with DOOM here. If they made the argument 20 years later, we paid you for work so we own the character, thats one thing. But these guys signed, at the time, specific contracts saying, any characters they created are owned by the company, and they were willing to sign it in order to get a job they really wanted to have....then thats on them.
There have been several notable people who decided, I don't want that deal, I am going to take the hard road and do it on my own, and I will own everything I do. That is a tough decision and I would say most who make that decision don't make it, because its hard to produce a comic and get it out to people (or was, it is much much easier to do it today in the world of digital and web comics). Those who wanted the fast road to success by signing on with the big team, they knew the deal they were getting upfront. It wasn't sneaky or underhanded, it was a contract.