Peter47. The regulation was brought in for one reason. A certain Irish LCC implemented a handling charge for PRMs. The Commission and the Parliament fearing a race to the bottom, decided to require that PRMs be provided with assistance without an additional charge. To ensure consistency they decided that the airports would be responsible - and that the airports could not make a profit, but only bill the airlines serving said airport on a per-passenger basis. i.e. If BA has 47*% of the passenger total at Heathrow they would pay 47% of Heathrow's total PRM costs. (*This is a guess, I have no idea what the actual number is).
As this Regulation was being developed the network airlines, at least, wanted to be able to decide if they would provide PRM assistance for their passengers. Their arguments failed.
This has worked almost everywhere in the EU, however some UK airports are well behind the curve.