Moulinsart is not a "foundation". The S.A. after the name stands for Société Anonyme
, or limited company. Moulinsart S.A. is the exclusive worldwide owner of all exploitation rights related to the work of Hergé.
You might be confusing them with the Fondation Hergé, which is a charitable organization created by Fanny Rodwell (who is the actual rights holder), to administer the legacy of Hergé; she formed Moulinsart to make the money necessary to preserve the archives, deal with the business of merchandise and publishing, and take care of the exhibitions and displays which tour. The objective is that the Fondation will create a permenant museum to house the collection.
As to their "criteria" for sanctioning works - if somebody approaches them with a commercial idea and they like it, they would probably do it. How they make those judgments is really down to them, and they are unlikely to say how they do it.
Moulinsart have published a charter which concerns the use of Hergé's work on the internet. You can read it here