I've managed to get 3x 10% on Day 1, and 1x 10% on Day 4 (just nice to get my PB Sasuke). But other than that, I agree with you that they should have more voucher opportunities. A chance for them to earn some money but they don't take the advantage.
This is what's happening:
They want to earn large amounts, so they think going big earns more if people spend. So its like a $10 meal, but because its $10, only 100 people spend on it, earning them $1000.
But if they use the same system and lower the price, so its like a $8 meal, more people will get it, maybe 150, earning them $1200. Looking at small scale aspect of 100-150 people. Seems like a small increase. But if you multiply that by the number of players and spenders, I think that's easily a significant amount of 20% increase.