Prior to the late 80s or early 90s -- let's say 1985, just to be conservative -- it was widely believed that pedophiles could be "cured" of their perversion. As a result, it was not unreasonable for someone to believe that they could send a pedophile off to treatment, get him "cured," and then be assured that everything would be OK.
That's what the Church did. Unfortunately, they also failed to tell the folks at the parish where they sent the "cured" pedophile priest next that he had these issues, or even actively concealed that fact. This failure by the hierarchy is what I find most disturbing.