Very short story.
I've been reading the Constitution of the United States the other day. Remember such thing? The one the regulates the foundation of United States, the government and such? Yes, that one.
So, I was reading it, you know, just to refresh my memory, and one thing just called my attention:
ARTICLE I.
...
Sect. 9.
...
The privilege of the writ of habeas corpus shall not be suspended, unless when in cases of rebellion or invasion the public safety may require it.
No bill of attainder, or ex post facto law, shall be passed.
So, unless our Congress really wants to violate "the Supreme Law of the Land", I don't see any legal reasons to pass the retroactive immunity to the telecommunication companies.
I may be wrong, of course. And I wonder why nobody even raised such objection to this immunity before...