JPII did not excommunicate them, they excommunicated themselves by their schismatic act(NB: I do not hold them to be in Schism, but this is how Ecclesia Dei has described the act).
However, if Pope Benedict has decided to lift the excommunications, it has not been without careful consideration. This has been a main goal(as it should be, with a group that claims over a million lay supporters). The SSPX has been saying this has been coming since back in November, when they started a Rosary Crusade for the intention(said to have totaled ~1,703,000 rosaries), and there have been several previous efforts to do the same thing.
To be honest, I may not fully embrace the SSPX the second they come back, but I will be glad to know they are back, so that they may once again be in full communion, and offer licit Masses, as well as valid marriages and confessions. They have a great zeal and a great passion. I do not agree with all they stand for, but then again, I find that with every Order I have looked at, which is why I discerned not to join each of them.