I don't entirely accept Nukecad's explanation. The free version of CCleaner installed without exception, as have other apps that I have purchased outside of the app store.
I don't really think this is a "Mac issue" it's a CCleaner issue. Just spent 30 minutes trying to figure out how to install the product I just purchased. Why am I paying money for this when the free version installs fine? CCleaner should do a better job of explaining this.