Stephens being where he was had zero influence on the fact that Charles' momentum was already taking him the other way. He didn't block his line of sight, nor did he prevent him from being able to get to the ball. What's more, he tried to get out of the way rather than move towards or attempt to play the ball. The word 'interfering' is doing some seriously heavy lifting here.
Even if we accept that he was, and that it is technically the right decision, I think what most people are rightly miffed about is the fact that neither the ref nor the lino saw any reason to disallow it initially, and only decided to do so after Charles and the other QPR players whinged about it. No competent referee should ever allow themselves to be swayed by player complaints like that.