I think the Dutch government have something similar to the UK Human Rights Act. This makes the rights defined in the European Convention on Human Rights enforceable in the Dutch courts. The parliament could change that law but unless it does the courts can instruct the executive in how to apply the law so as to maintain those rights.
A quick google also suggests that international treaties which the Netherlands has ratified may be directly enforcable in the Dutch courts without needing additional laws to be passed implementing them (as would be the case in the UK). This seems to be a confused area, but it sounds like the court is taking this line.