I have been trying to put into words what I think might be wrong with this hypothetical -- I'm not even ready to ask about a halachic answer because I'm not clear on what the halachic question is:
I went out to eat with someone else. The bill was sizable but instead of having us split the check and submit 2 credit cards, I volunteered to put the whole bill on my card and have him pay me back.
My motives were not altruistic. If I pay on my card, I get points/miles. The more I pay, the more I earn. But in this case, he is paying me back, so I am, effectively, being rewarded with points/miles for a payment that, in a sense, I am not making. [I could accomplish the same by wandering around and volunteering to pay everyone's bills as long as they pay me back.]
From a halachic stand point, am I defrauding the credit card company, taking advantage of my friend (would it make a difference if he knew my motives?), or somehow else taking interest or engaging in unfair business practices?