As far as I know, that is a standard practice; they routinely check your credit report to make sure your creditworthiness hasn't declined.

They oftentimes use a drop in your credit score to invoke the universal default clause of your cardmember agreement - which allows them to raise your interest rates at will.

There have been many stories done about how Joe Schmoe was late paying his Citi card - AMEX finds out he was late and raises his interest rates to the legal limit allowed by law. In AMEX's eyes, he became a higher credit risk (even thouh he may pay them on time), and by law they can raise your rates.

Ethical? Maybe not. But legal nonetheless. More about universal default can be found (of course!) on Wikipedia.

http://en.wikipedia.org/wiki/Universal_default