Of course, even that's flawed, given the way the questions were structured. No matter how you do it, you're comparing two different measurables: "interested/uninterested", and "1-5". So even in your case, you'd be measuring how "uninterested" turns into a numeric scale if snacks are offered...and what the hell kind of result would that give? "Everyone who was uninterested, when offered snack foods, would have had their opinion influenced by a measure of 2 on a 5-scale." Better to ask the second question "Would you have been interested or uninterested if snack foods were offered", so you can actually measure the effect the offer has (i.e. "23% of uninterested people were interested if snacks were available".)
And then, of course, there's the little fact that "intent" and "action" are two different things. If you're studying how offering snacks affects membership in a club, your measurable should be membership, not intent...