>Getting such a law passed does nothing to prevent a future law from saying the opposite.
What it does is make the proposal for the future law look like a much larger departure from the status quo, which makes it a harder sell. Furthermore, members of Congress don't like to change their positions for a number of reasons relating to both ego and what it allows election opponents to put in political advertisements, so if you can get them on record supporting your cause then you make them less likely to go against you in the future.
EDIT: Another option is for the courts to decide that freedom was guaranteed in the Constitution all along. But courts are unpredictable so again, good luck!
