If she comes back to the UK to stand trial it is imperative she is kept in a secure unit until it takes place. As far as I am aware she has never voiced any regrets for her decision to join ISIS.
People who are sticking up for Begum might not be so forgiving if she commits an act of terrorism, which kills or maims one of their loved ones.
While currently the court has ruled that she should be brought back to appeal a ministerial decision about her citizenship status. It isn't clear that there is a criminal case to answer, If she comes back and there are no criminal charges, and it's the citizenship cases, then locking her up would be an abuse of the criminal justice system.
I'm not sticking up for her, I'm sticking up for treating het with due legal process. People who want to subvert that because of their 'opinion' are very scary.
As the quote from A Man for All Seasons goes:
“William Roper: “So, now you give the Devil the benefit of law!”
Sir Thomas More: “Yes! What would you do? Cut a great road through the law to get after the Devil?”
William Roper: “Yes, I'd cut down every law in England to do that!”
Sir Thomas More: “Oh? And when the last law was down, and the Devil turned 'round on you, where would you hide, Roper, the laws all being flat? This country is planted thick with laws, from coast to coast, Man's laws, not God's! And if you cut them down, and you're just the man to do it, do you really think you could stand upright in the winds that would blow then? Yes, I'd give the Devil benefit of law, for my own safety's sake!”