Actually, the Mage Tourney law was that you couldn't use non-color magic. So you could use Blue, Black, Red, and White magic, but no other types.

But the laws in A2 work differently. They apply only to your clan, and they only affect a couple things. First, you have some abilities called "Clan Priveleges". You can pick one of these per battle, and they give you some minor boost (some boost attack damage, others evade chance, etc...). If you break the law, you lose the Privelege bonus. Also, when characters die in battle, they vanish from the battle instead of staying as a corpse. Raise magic or a Phoenix Down can be used to revive them, but you can only revive a character if you have not broken the law. And, finally, if you don't break the law, you will receive 3 items at the end of the battle. Usually one minor item, like a potion, one piece of loot, and one basic piece of equipment.

So, if you don't break the law, you get a few extra items, a little boost in your battle abilities, and can revive your fallen allies. But your allies won't be removed from battle the way they were in FFTA, and very few missions require you to follow the law.