Man, I know this shit wouldn't fly in the US. The US government has its problems, but at least it seems to be well aware that it's not allowed to claim copyright on any information it publishes. For example, lots of government-produced data has wound up on Wikipedia, and the US government is not freaking out about that. (Good for them.)

I suspect Belgian law has a similar provision, and some idiot lawyer either doesn't know that, or is hoping to scare tuinslak into not competing with the government's site. (Or is it conceivable that Belgian law actually permits this C&D?)