Why do you think that neither house of Congress is allowed to amend conference reports?

In order for a bill to be presented to the President for signature, it must pass both the House and Senate in the exact same form. The device used for reaching agreement between the two Houses is often, but not always, a conference committee. Sometimes differences between the two bodies are resolved by amendment - e. g., the House will agree to the bill as passed by the Senate with an amendment and the Senate will subsequently concur with that amendment.