Thoughts on Megan's Law and the free movement of violent criminals in Europe

Today we heard a minister is going to the US to study Megan's Law in view of importing it to the UK. This was welcomed by campaigners such as the mother of murdered school girl Sarah Payne after whom the proposed law is already being named: Sarah's Law.
Also, the BBC aired a programme about sexual offenders from other European countries having served sentences in their countries and being allowed into the UK only to re-offend.
What is the logic of Megan's Law? The idea is that people will be able to find out at the click of a mouse if a convicted and released paedophile is living in their area. How is this supposed to protect children? Is the responsibility of the police for protecting citizens, including children, to be handed over to ordinary citizens? Are parents supposed to take the law into their own hands?
Is it not our right to believe, that once an offender has served his or her sentence, then they should no longer be a risk?
As for offenders from european countries, if we do not believe certain countries are able either to reform their offenders, should we be allowing their citizens in to the UK freely? Perhaps this shows the meaninglessness at present of Europe as a community.