The radio news is full of MPs bleating about how unfair it is that the rules on reasonable expenses have been changed after the fact. It's called retrospective legislation chaps - just like the rules that have been introduced by this government that put us ordinary mortals at a disadvantage. Tax laws are replete with retrospective changes to close loopholes introduced by the poor drafting of legislation in the first place - for example, the infamous (if you are a contractor) Section 58.
Funnily enough, this page makes almost the same the point I was going to:
"One former contractor ... commented 'How would some Cabinet Ministers feel if the rules on MPs expenses were changed retrospectively and they were forced to pay back all the money they had claimed for so called second homes? Apparently, it is OK for them to use loopholes in the rules but it is not OK for the rest of us.' "