There's 2 things really. First, nobody should be jailed purely as a prisoner of conscience, even people with abhorrent views. It's not wrong for the government to urge the release of such prisoners. Second, the UK government, of all flavours, should have done some due diligence to ensure they didn't look ridiculous by celebrating the release of a man who calls for violence and, having arrested others for less, leaving themselves with the problem of looking like there's a two tier system.
I see he's got his apology out pretty sharpish though so he probably won't be getting prosecuted. There might a polite police visit and a statement about the tweets being youthful, historic indiscretions.