Well you assume it's obvious what they've got to hide!!
Interestingly though, when I worked for the borough staff above a certain salary level were required to sign a form each year declaring any memberships of such organisations. I knew of at least 2 who didn't declare they were Freemasons. When I queried this with one of them (actually when he was trying to recruit me!) he said as far as he was concerned it was none of the councils business. He was prepared to declare other things but not the Masons.