Why do so many silly ideas become law?
Most of the politicians I know have exceptionally good memories, and studied history. So why do they repeat the mistakes from former years? Perhaps because they've learned the lessons that apply to them, and the mistakes that apply to the general public don't apply to them.
A perfect example of this is seen in both the public sector and the commercial world. A manager or leader has built their reputation on always balancing the budget. They are sent in to troubleshoot failing divisions: they cut spend immediately, usually by terminating all the R&D, and everybody praises them because now costs are lower than income. Then they move on to the next piece of troubleshooting.
And what happens?
within a year or two, the lack of R&D means that this organisation's outputs (services or products) are no longer fit for purpose. It falls to the next manager to take the blame -- the previous self-serving manager retained only the glory of having balance the budget. Of course the previous self-serving manager is also great deal more senior, and is therefore in an even stronger position to make sure the blame lies elsewhere.
You can achieve the same result by terminating overtime, cutting staff, and making everybody work harder. It takes a little while before everybody goes off sick, and people start resigning and finding other jobs. The canny self publicist is well aware of this time-lag, and has moved on quickly. A very clever self publicist will identify where someone else has just done the right thing, and unseat them in time to reap the rewards.[reference available in Freakonomics]
How long does it take for the pigeons to come home?
Typically the effects are visible between 18 months and two years after the change. That's why I call this the two-year rule.
Look around you -- how often have you seen this happen?