One of the basic principles upholding a free society is that the law should be known. This is subverted when retroactive legislation is passed. No person can know they are breaking the law if the law has not yet been passed. In recent years parliament has taken to making some of its laws retrospective. This was originally used against suspected war criminals, but has more recently been incorporated into finance bills to close loopholes the Treasury calls tax dodges.
The principle should be that whatever a person is accused of should be a crime at the time they committed it. How can people keep their behaviour within the law if the laws are only enacted afterwards? Retroactive legislation is another case of the steady erosion of liberties by the subversion of the principles which sustain them.