They don't need any reasons. The software is third party and affects the client by overlaying a HUD. This is not something other users of PokerStars may have access to thus can be considered an unfair advantage. They need to protect their players and their product.
You can continue using the product and risk being banned. Or you can stop using the product and run no risk.
In regards to purchasing the product you have no option there as purchasing products like this always run a risk or them not working with the client you choose or for the same situation you ran into.
Ad Toby stated it seems to be a temporary issue. I did a little research as well and it seems PS changes their rules from time to time which makes older clients bannable. When using a third party program make sure you stay in the HM forums and PS forums to stay up to date on the software and the rules.