"Literary merit aside, 50 Shades of Grey was a massive hit, and also pornographic, meaning that lots and lots of people read it,"

Which is probably something that the UK government does not want happening again.

"and lots and lots of people are now "criminals"."

No they're not, unless they decide to share copies of the novel online. I must have really missed it if there was something about retroactively enforcing laws.

There are legit concerns here, but fsog isn't one.