I think they want Roman to be a babyface though. I personally feel he is a better heel though.
Yeah they do want Roman to b3 a babyface but a lot of fans don't like Roman because they feel like WWE is pushing him and trying to force them to like him. This is why a temporary heel turn could actually help Roman overcome that barrier with fans. A lot of the fans who don't like Roman simply because they feel they're supposed to, would warm up to him if he plays a heel role that could gradually transition back to a babyface.
It's the same scenario that worked for the Rock. WWF was pushing him as their next big star and many of us, including me, resisted and ended up hating him (and God, Rocky Maivia was a shit character) but when we were supposed to hate him, we found reasons to like him.
So a temporary heel turn for Reigns could make him a better babyface in the future. It's not guaranteed, but simply wanting Reigns as their top babyface isn't a reason to not turn him heel.