No one is saying that a safety pilot acting as PIC in the US isn't PIC. It happened in the US, under their rules and the Feds recognise it.
What they ARE saying is that because someone else has also logged PIC, which is inconsistent with EASA rules, there is now doubt as to who gets the PIC credit. To accurately credit PIC hours, you would need to supply your logbook, and the logbooks of every safety pilot you have flown with, which would all need to have accurately recorded who was 'acting' and who was not (a distinction not likely or required) - so that they could assign PIC and SNY. Clearly this would be completely impractical, so the simplest solution is to simply disallow (for licence issue) the relevant hours.
Put simply, a flight has to have a PIC, and the safety pilot would be deemed to be the PIC. The problem is in proving it.