It's true, cats don't taste sugar at all. Carnivores have no need for taste buds that detect 'sweetness', so they don't have them. That's why a cat will turn its nose up at chocolate.
Floid, yes, my cat is getting fat on these biscuits (but not quite Tubcat yet ;-)). But she simply won't eat anything else. You don't know how annoying - not to mention destructive - a cat can be when it doesn't get its own way. Short of locking her up in a wooden chest and burying her in the garden for three days, I'm not sure what actions I can take which won't result in frayed tempers, carpets, and wallpaper. I could give her less of the biscuits, of course - but I want to vary her diet. She doesn't, apparently.