It is a chain effect. Direct target heals(*), reduce rage regen, which for a wd/w reduces the use of rage skills(ie slash), which is a dps generater. Now if they are doing a tanking role, it ends up affecting aggro generation in that chain too.
It seems that players are either so OP, or used to it already, that it is transparent, which is all a really good thing. This is mainly for players that want to improve or do not know about the link between heals and rage regen.
* examples of direct target heals are:
druid-Recover, Restore Life, Blossoming Life
priest-Urgent Heal, Regeneration, Heal