Insurance companies price insurance based on a driver's risk rating - any "fault" raises the driver's risk rating, so he/she pays more. However, any claim, EVEN when the driver is NOT at fault, raises the driver's risk rating - though not by that much. The insurance companies' argument is that on average, risky drivers are involved in accidents more than safe drivers, regardless of who is at fault in a particular accident (to some extent, it is true!!). So ANY claim leads to a rise in premiums. Same thing happens with home insurance - you file the most innocuous of claims, and the premiums go up - there, even the argument of risk rating doesn't hold here(are you a "higher risk" homeowner if a storm damages a window in your house?).
So either it is legal to increase premiums regardless of who is at fault, OR insurance companies have been scaming tons of customers for a while (and that is certainly possible!!). I have seen it happen in at least 3 instances.