I looked at the checkbox in dev tools and found lots of event listeners attached to it. JCE is still using mootools?
Anyway, removing those listeners didn't make a difference either.
All other checkboxes are working fine, only the Preview Tab is zombied.
I use a plugin to remove mootools, I only see it loaded on the JCE profile pages. When removing the mootools files, the profile menu disappears. Therefore it seemed to me, that JCE still needed mootools ...
EDIT: Checked again, mootools is actually stil present in other places, too. Bummer.