What's the point in waiting 6 or 12 months before allowing researchers to make their Belgium-funded article open access?
I think you are reading this wrong. It's not that you are not allowed to make something open access before this time span, it's that you have to after it when public money has been used. This is called Delayed Open Access (thanks to Anyon, who mentions this in a comment).

Specifically note that the journal is free to open up the paper earlier:


  The publishing contract may provide for a shorter period than that set paragraph 1.


(for instance, if you paid the open access fee). I think the most salient point is this:


  The right provided for in paragraph 1 can not be waived.


That is, you cannot sign a copyright transfer to a closed-access journal that gives them the right to not make it open access even after the closing period is over.

If you are asking why they don't require immediate open access, you are back at Najib's answer - publishing is a business, and presumably the Belgian legislation has decided that Delayed Open Access is an acceptable compromise between the publisher's interests and the public's interest in free access to scientific information.