Saying that cats can't taste sweets because they lack the proper genes for sweet receptors is just theory. We don't know and can't know what cats taste. We CAN observe their behavior and many cats definitely show an affinity for sweet things by displaying the appropriate behavior. So, although something sweet may taste entirely different to a cat than it tastes to us, I firmly believe they are sensing something pleasurable because that's the behavior that results.
As an example, Rocket hates tacking liquid meds. Yet, when I had to give him a liquid med via syringe, a med which contained lactose and tasted sweet
, he liked taking it and slurped it up from the syringe like I'd slurp up a chocolate shake.