This has been a known limitation of Google Play Movies since launch. It will not play on a rooted device. Don't blame Google, place the blame where it squarely belongs with Hollywood lawyers and dingbats who do not understand technology.
I'm not sure if it will work for Google Play Movies, but I had the same issue with playing movies through the Fandango app and Voodoo OTA Rootkeeper app did the trick. It "temporarily unroots" your phone while you need it to be unrooted and then you re-root with the push of a button. Search for it in the market.