According to an Android Pay engineer on the XDA forums, Android Pay is not compatible with rooted devices because of security concerns: on unrooted devices apps run in a sandbox and can only obtain limited information.
How is this different on an unrooted device as apps must request permission from SU?
Could an app trick SU into giving it more permissions than possible?
I am interesting in getting Android Pay working on my phone but have concerns about the security of my data as my device is rooted.