[Starknaked]

It's not the user's responsibilty to ensure these incentives are sustainable. Look at what happened with the Hoover free flight program.

[IanCal]

The user is the one, well, using it! Sure, in some cases it's not obvious if you're abusing the system but this seems pretty clear.

[Starknaked]

The best way I've seen businesses get around this issue is to just have an upper limit on the incentive like interest applies only to the first $x. Its often a nice bonus for people that were going to use them anyway but not enough for people to justify exploiting the incentive alone.