Optional health insurance questionnaire - where's the catch?

My employer is “restructuring the employee health plan to offer better benefits”, which invariably means they’ve located another way to make us pay more for less, no news there.

So, looking through the paperwork trying to figure out exactly how they’re going to cloak the bad news, I come across this amazing offer of a gift certificate if I join up on their website and complete a health questionnaire. I know of some entities in this world who’d give something for nothing, but health insurance companies have so far failed to make that list.

So, what’s the catch? It’s a decent enough gift certificate (>$100), but it also looks like a pretty intrusive questionnaire. Am I being paranoid (damn, now I have to look into mental health coverage) or just cautious in not trusting Greeks, even when carrying gift(card)s?