Why?! c2it aren't doing it to give them a warm, fuzzy glow. They're doing it to get more people into debt and to control a market.

This is like saying that it's immoral to pay off your credit card on time. Credit issuers give us interest free periods and frequent flyer points in the hope that we will over-use our cards and do into debt.

Are those who always pay their cards off on time being "immoral" too? How about those who go into their supermarket and only buy the loss-leaders that are on sale and don't buy any non-discounted products?

As has already been mentioned, it is normal for the rules to prevent infinite loops, such as for paying for, or payments by, gift voucher not to receive rewards. Supermarkets often make arbitrary quantity limits. One of my credit cards limits balance transfers (which earn reward points!) to 6 times per year, so that 6 times your credit limit is the effective max. per year.

If a company comes up with rules which allow something, I'd say fine, use it. If they are too incompetent to set up their rules sensibly, maybe they should employ one of us to write the rules for them?