Diet Coke is healthy, you say? "They'll get my artificially-sweetened, carbonated, caramel-flavored beverage when they remove it from my cold ..."
Seriously, never underestimate the power of killjoys. Call it the Domino Theory applied to personal behavior.
You're absolutely right: the fatty goodness of fair food is what makes it appealing. A summer isn't a summer without at least a big serving of an elephant ear or funnel cake, though I draw the line at deep-fried packaged foods such as Twinkies or Snickers bars.