For those who don't want the ISIS girl to come back, presumably they don't agree with the rule of law. Her right to return has nothing to do with her political beliefs, but everything to do with the law. If the law says she is a British citizen and therefore has a right to return, then that is the law.
What people might argue is that the law should be changed, or that she should be subject to certain conditions, treated in a certain way when she does return, should she return etc. Or that it is a stupid law. But her right to return is a question of law.
On the issue as a moral issue. Is it right that the government should determine that someone knows their own mind sufficiently aged 15 to make the decision that she did and then be denied the right of citizenship as a result yet determine that at the same age she wasn't old enough to know her mind sufficiently to vote? Or what about UK citizens that serve as mercenaries in foreign conflicts, should they be allowed back? What about mercenaries fighting in Yemen on the Saudi/UAE side? Or other African conflicts?
I'm not suggesting that what the girl did was morally right - i don't think it was - but more that situations like this are rarely as binary as opinions on social media might suggest.