It depends I think on a few things; one, finance. I think he's making plenty of money with his books and his career at the university, so I don't think that's a problem for him. Two, morals. If Minix was used in e.g. ballistic missiles (which it might just be) I'm sure he'd have objections - even if there's nothing he could legally do about it due to the license, as acknowledged in this letter too.