The Senate is able to do this little maneuver because its rules, unlike the House rules, do not require that an amendment be "germane" to the subject matter of the original bill. In addition to the shell bill trick, it is not uncommon for all sorts of non-germane amendments, which wouldn't have a chance on their own, to be stuck onto some must-pass piece of legislation. Some of them get weeded out in the House-Senate conference committee, and some do not.