Article 1, Section 9 of the U.S. Constitution--you know, the one they swore to defend--says, "No ... ex post facto Law shall be passed."
How would it be constitutional for the Congress to pass a law that makes an illegal activity suddenly legal for a certain class of persons? And how can you do it without it becoming ex post facto the very instant you did it? And if they give a pass to this type of activity, what will they allow next time?
The last thing we want in this country is a precedent whereby the government runs roughshod over our rights by proxy through private companies. Right now, we're setting up a situation where when the you-know-what hits the fan, the executive branch gets their buddies in congress to pass a law granting immunity to private companies who did their dirty work for them. This, of course, strips the people of their expectation of accountability and equal justice under the law.
I only see chaos and tyranny down this road.