Why not prepaid utilities?

Just thinking out loud again...why don't utility companies (or grocery stores for that matter) offer pre-paid cards? It'd be nice to give someone a gift of $$ off of their electric bill or next weeks food supplies. In fact, considering that many companies have debts in business, they could offset the interest they pay on these business loans with pre-paid customers. Let's say Dominion Electric Co. has a loan out for $10M at 8% interest, and instead, offer their customers 5% off on pre-paid electricity? Among those people who are capable, who wouldn't invest in next month's electricity if they could get 5% on their money?