I agree there’s a measure of complexity in interpreting the phrase – but not in the area you seem to think there is.
There is not any interpretation of the ex post facto clause that remotely comes close to forbidding a present grant of immunity for civil liability that covers past conduct.
The complexity to the ex post facto clause arises when considering things like a changed rule of evidence. No one has any trouble with the concept that Congress cannot pass a law making mopery illegal on July 1st, 2007, and the federal government follow that by prosecuting you for mopery committed on June 26, 2007.
But – what if Congress changes the federal rules of evidence on July 1st, 2007, to make admissible certain types of evidence that were not admissble before, and then the government decides to prosecute you? What if the change comes as the result of a Supreme Court ruling about admissibility of evidence?
Interesting stuff, which I’ll be happy to discuss in detail if anyone’s …er… interested. But nothing to do with civil immunity for past conduct.
NOw, that’s not to say that there might not be constitutional implications. In the past, Congress has paired the immunizing of tortfeasors with a statutory compensation fund – the 9/11 fund, for example. Congress said, in effect, here’s this money, but in exchange you can’t sue the airlines. In other instances Congress hasn’t given victims a choice – the asbestos legislation immunized companies against asbestos claims, created a federal compensation fund, and didn’t give victims a choice of opting out and suing.
But is that a matter simply of wise public policy, or is offering some sort of compensation when you take away the right to sue a constitutional requirement?
Probably not. Consider the famous case of Fisch v. General Motors. This was a case that arose from the post-WWII “Fair Labor Standards Act.” After Congress passed the law, the courts interpreting it found that it required a company to pay a worker from the moment they entered the factory, rather than (as had been the practice) only from the moment he actually arrived at his station and began work. These decisions created a veritable onslaught of lawsuits against companies who had been doing it the old way, and Congress reacted by passing a law eliminating the liability after the fact, which law was then upheld by the federal courts.
Obviously, this is not precisely on-point. But I am aware no case law that holds the reverse, nor of a viable constitutional theory that would forbid Congress’ action here.