Where does it say they provide the raw feed? Everywhere I look in the Facebook help, terms and privacy policy it mentions how they use algorithms to determine what stories appear, and how they use information provided by users to pick stories. They also mention using user information for internal testing and analysis.
They seem to be following those terms, they were choosing positive and negative stories for feeds, and then analyzing the data to see if users then posted more positive or negative posts in return.
1. '...we may make friend suggestions, pick stories for your News Feed, or suggest people to tag in photos...'
2. 'The News Feed algorithm uses several factors to determine top stories...'
3. 'How we use the information we receive... for internal operations, including troubleshooting, data analysis, testing, research and service improvement.'
The UK is free to look into the Facebook experiment and to pursue the case. That alone doesn't make Facebook guilty of anything. They didn't go to a court of law, Facebook wasn't proven guilty of breaking the UK Data Protection Act, they're just investigating whether proper precautions were taken.
Facebook should obviously be bound to the same laws as everyone else. Which human experimentation laws did they break? Users registered on Facebook and agreed to the terms of service, and how Facebook will choose which stories they view, and analyze their response.