Why do you think that everyone spends extra hundreds per year to get a free flight? Most people I know that use rewards cards use them because they put most of their expenses on credit cards anyway so getting an extra 2% back is just a nice perk.
The study compared credit cards and cash. I suspect the difference is related more to the _card_ aspect than to the _credit_ aspect, as the former takes away the 'did I bring enough cash with me' issue.
"McDonald's found that the average transaction rose from $4.50 to $7.00 when customers used plastic instead of cash."
I have a certain amount of money in my pocket. If the bill is higher than that amount, I will definitely pay by card. If it's lower, than I'll choose card or cash. So, even if the card/cash choice had no impact on my spending patterns, you would expect my cash transactions to have a lower average value.
