"I think it's up to the Netherlands to decide whether they want ISIS supporters and consequential haters of the west, in thier country."
In normal circumstances I would agree. However, members of the EU do not operate on what I would call normal circumstances. Once in Holland they will have freedom to roam all over the Schengen Area without checks. As well as that they will have the right, as EU citizens, to travel to the UK where denying them entry could prove very problematic.