I believe this has been in every credit card agreement I have gotten... they have the right to credit back points you received. I have always seen it balanced each month... usually I have enough new charges to balance out any credits during the billing period. However, I recall two instances... once with Amex and once with Chase, where I had a negative balance at the end of the billing period. In both cases, they took points back from my account.

Seems totally fair to me....