I don't think that the Compendium comes right out and says this, but while mowing the lawn tonight it occurred to me that there is more that could be said about why this is an essential human right.
Man's ultimate end is to see God: the beatific vision. But if this is the case, then it would be gravely evil for anyone or anything to obstruct a man's ability to attain that end.
That is the negative side: a man must not be prevented or hindered from achieving his last end. But what about the other side: compelling a man to believe in Jesus Christ? That fails by virtue of the previous argument: religious belief of necessity must be a human act, or it is not really belief. So if we force a man to be baptized, he would not have the necessary disposition required to receive the grace promised in the sacrament, and consequently it would be of no benefit to him anyway.