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.

A good argument for reinstating the President's line item veto power.

Though of course, that particular sentiment might be contingent upon who you are talking to, and who happens to occupy the White House at any given period of time.

A pity that there's not a constitutional amendment forbidding non-germane amendments from being incorporated into a piece of legislation.

It would weed out a lot of trash that often gets put into a bill.