The problem is that whilst we know that helicopters can cause wake vortices, little or no research has been done on how helicopters are affected by these vortices and the turbulence they create and how well they can resist those effects. As such, the wake turbulence separations are applied equally to both fixed and rotary wing.
And that seems to be the problem - no-one wants to listen to helicopter pilots who know the wake turbulence from a FW just isn't a problem - downwash from another helicopter is a different matter and we all know about how to avoid that.
Even the MATS pages (thanks for the links) specify in the opening paragraphs that wake turbulence vortices are most hazardous to aircraft with a small wingspan during the take-off, initial climb, final approach and landing phases of flight - nothing there about affecting helicopters at all.
It just seems a very lazy option to apply FW limitations to RW operations.
If you are trying to complete an instructional sortie within an allotted time (because the customer is paying) and you are constantly held and delayed for wake turbulence, it is a frustrating and inefficient restriction of airspace, but it seems the controllers hands are tied because the authorities can't be bothered to acknowledge that helicopters are different.
I am led to believe that the FAA don't apply wake turbulence separation to helicopters on approach behind a FW.