They probably believe that their target market (i.e. donors) would consider it an advantage if their offspring would study together with people with "good linea

They probably believe that their target market (i.e. donors) would consider it an advantage if their offspring would study together with people with "good lineage" (e.g. children of foreign leaders) - and I don't think that they're mistaken, I think that they likely know their market characteristics better than me or you.