The Daily Mail gives one explanation, in a piece not online:

Her aunt, Wendy Hagenbuch, told the Mail last night: ‘Kate will be very forgiving of these men because she believes they wouldn’t do this sort of thing without good reason. She has a very sympathetic nature’.

Kidnappers, of course, are known for always acting from good reason.

UPDATE: Melanie Phillips, backs from hiatus, adds more, pointing to the website where Kate Burton does her "very important work", which presents all violence in the area as initiated by Israel, the vile oppresser, while Palestinians actions are whitewashed. Palestinian suicide bombs are "protests, etc.

Phillips highlights the charitable mindset of the extended Burton family, and then points out how this very British decency becomes upended on the subject of Israel.

This is what is so distressing about what has happened to Britain. Nice people now turn out to have monstrous views. Decency itself has been kidnapped and brainwashed and turned into a diabolical mirror image of itself. The moment you meet one of these liberal, tolerant, well-educated, well-spoken, well-mannered, internationalist-minded folk for whom the third world is a synonym for global injustice, you know that they are going to despise or hate Israel and bestow their compassion on the promoters of genocide. One constantly meets such people who have compassion for the vulnerable and want to do good in the world: pillars of the community, admirable and delightful in every way — except that they side with people who happen to have a murderous hatred of Israel and the Jews.

UPDATE II: And even more wisdom from Ms. Burton in the BBC:

They said they had carried out the kidnappings to highlight issues such as Palestinian prisoners being held in Israeli jails.

She said she believed they would have used a different tactic if they felt they could.

"I got the feeling that there was a real sadness in the kidnappers' hearts that they needed to do this."

Didn't we always know that this would somehow be the fault of the Israelies!? Imagine! The injustice of putting Palestinians in prison for terrorism and murder.

UPDATE III:

The kidnapping had made [Kate] "love the Palestinian people even more," she said, adding that the majority were peace-loving and disapproved of violence.