Thank you for contacting us.
In current implementation you should re-apply the button's template in order to change the background. It is due to a combination of the binding and the visual state manager action. We will correct this one - probably in our next internal build. However with the current release you should follow the technique demonstrated in the attached example. Please give it a try and let us know if you have other questions.
PS: Actually your question is an issue-report and because of this we updated your Telerik points.
All the best,
the Telerik team