I think it's the right choice to reward a team with 8% damage for taking a strategic advantage. No matter how you look at it, getting dragon is the result of taking a strategic advantage whether it is vision advantage, tp advantage, or taking a dragon when the bot lane recalls. The fact that people are losing the games when the first dragon is taken by the enemy team is probably because the enemy team is playing a better team game. I have personally won several games where we lost first dragon and when the enemy team had 5 dragons. 8% damage is not enough to lose the game, but it's enough to be able to push an advantage you already have.
If you move 8% damage to the 3rd aspect of dragon, the first two dragons really don't have the same weight on them. You are making strategic trades of maybe a tower or 2 and map control for 8% damage. instead you'll be trading a tower or 2 and map control of that area for 15% damage to minions? i wouldn't want to trade that.