It's probably just as difficult to monitor the man's every move. I doubt that the police can follow him every day to make sure he is not near a playground, school or ball field 24/7.
In a public library you must show some ID to use a computer, so local libraries could be alerted and deny him use. It is possible, I suppose, to find out whether he is using an internet provider. You couldn't really stop him from purchasing a computer or laptop since that would be next to impossible.
Like any condition of supervised release, this one would be enforceable only up to a point.
But, is it really fair to the man to deny him all internet use, particularly if that would make him unemployable?
If he made obscene phone calls would you deny him all use of a telephone? If he sent or received child porn, would you deny him all use of the mail?
I suppose that's the crux of the legal dilemma.
I read a bit more about the case, which explains the judge's ruling.
http://www.law.com/jsp/article.jsp?id=1105364089680