Ability to Pay

Why do the Liberal Democrats believe that a local income tax based on ability to pay is a fundamentally fairer system than the one currently in place? If I go to Tescos, the price is set by the market and that's the price I pay. They don't ever say, "Snafu, you have a greater ability to pay than that pensioner over there, you should pay more". So why should the charging of council services be so very different?

Taking the analogy further, as the pensioner doesn't have to pay for her own food, no income you understand, she could quite easily spend all careering around Tescos loading her trolley with plenty of everything. Poor old Snafu then comes along for a loaf of bread and a tin of beans and the price is £1,000. A double whammy of ability to pay and that trolley that did the rounds earlier in the day. Yep, a far fairer system!!