... for driving a coach and horses through some basic principles of English Law feels equally unmade."
The case was, of course, *never* made in the first place, just a "precautionary principle" argument that "well, this material might cause problems and we don't like it, so let's ban it".
Meanwhile, however, we still have a book-burning law which says "if you've got anything that might fall under this law, you'd better delete it youself, just in case we nick you for something else and we use it as an easier way to get a conviction".
As for our worst fears not being realised, you miss out the important word "Yet". It took some while for Councils etc to realise the power that the RIPA gave them, but when they did figure it out, abuse of these powers rapidly became widespread.