No, the UK hasn't just prosecuted one. That was just the example from a couple of weeks ago.
Different returnees are dealt with in different ways, according to what seems most effective in preventing them (individually and en masse) from becoming a danger. I assume preventing terrorist attacks is an aim we're all agreed on?
There's debate about which means are most effective, especially long term, and what might end up having a negative affect, aiding the terrorists and boosting terrorist recruitment. But that's about effectiveness, not "bleeding hearts".