This logic is not going to be acceptable for any Board even they are taking some good amount from IPL every player is surely going to accept national duty as a priority because they have this just because of their country and money is never been a priority for many players just very few can go with this logic but mostly still will prefer national team on this IPL because here they have just one or two seasons and the national team is surely having some good long time for them.
Hopefully, in near future, we will have some better laws about this because no board will give this acceptable even they are making some good money from IPL if we have any confrontation in this than most boards will try to ban their players from joining this IPL because with this it's going to be a bad impact on this game. Sometime back same issue happened in soccer then FIFA gives a ruling in favour of countries, so I feel most chances this will happen here as well.
I would say majority of players are after money once they get recognition by playing international cricket from there country. I do agree with you that there must be some laws for every player regarding his availability for national team and playing leagues. The one whose priority is league must not play for national side from the day very beginning and we will see which league will include such player.