I don't think I consider a 20% annual percentage rate to be usury. Especially since it's 100% voluntary. You pay in full, you don't get charged interest, if you don't use the card, you don't get charged interest. I don't find that rate to be a problem at all.
And the recent CARD act fixed the major problem that I saw with credit card companies, in that they could raise the rate on your existing balance. Now they can only raise the rate on your future purchases.
If someone is paying 20% APR they likely don't have good credit. That same person will be paying a 14-17% rate for a used car loan or a 7-8% rate (with a significant down payment) on a home mortgage. For the car and mortgage, the bank can TAKE these things from you. That is a significant deterrent to not paying your bill. It also allows the bank to recoup a significant amount of what they lent.
If you want to be really shocked.....a pay day lender typically charges rates APRs in the 100's of % (sometimes 1000's of a %).