I can't see any legal implications in giving a negative report, if that were a problem then surely theatre and restaurant critics would forever be in court.
Could a woman who accepts cash for sex really sue for a report being "obscene".
To jump on my soapbox, I hope I answer the question:
Obscenity is subjective. Some people may have thought the film 9 Songs is obscene, some may think the books by Belle du Jour are obscene. If something is reported to the DPP and in the case of 'girls scream aloud' the IWF (internet watch foundation), a landmark court case followed. I think in the 21st century very little has been found as obscene.
Considering the cost and time and publicity a court case generates most people will not bother.
With regards Twitter I am not an expert, but the internet has gained popularity over the last few years with broadband, if anyone has used an old dial up modem they will tell you how slow it was at downloading\uploading pictures, videos, music etc (it took around 20 minutes to download 5MB).
Now the government realise like videotape in the 70s\80s the internet is not going away so they are looking to control it and Twitter, Facebook etc are all viable targets as they are used by billions of people and they have had some success with prosecutions (mainly for libel - the super injunctions etc). In the future copyright and pornography will be tested - there was an opt bill which so far hasn't passed to my knowledge - where the users signs up for pornography otherwise it is automatically filtered by ISPs.
So simply the government want to be seen swinging their "obscenely" small dicks around, because they are dealing with something they can't control. The DMCA is an American example where Bush Jr did some corporate lobbying (illegal in the UK if you are an MP, but Cameron did it with the Fox network), where he took a "kickback" to draw up a draft (or should that be daft) legislation - you'll see more of this in the future.