After some thought on the
Manwë stumbles thread (And
nice-ish sized post on there- maybe not long or explanatory enough, but it gets the point across), I think the lesson might be aimed at Ulmo. After all, he is too kind to his chosen people. Through Tuor, he created Numenor, and maybe Manwe wanted to show him that he shouldn't be too forgiving, otherwise his kindness would be abused, as in the case of Manwe himself with Melkor (or was he Morgoth by then?). Just another, less likely idea.