Yep, big fish eat little fish. Get over it.
Here's the main problem, which others have probably already mentioned: if higher level THs can't attack you, then you'd be able to climb to a very high number of trophies as a TH7 or 8 way too easily without ever really getting smacked down by the big boys. In short, your suggestion would completely break the entire trophy system.
And besides, what's it matter if you get 3 starred by a TH9 with Gowipe, or a TH7 using drags or giant wiz healer? Hint, the answer is you lose almost twice as much loot to the TH7, and since both are perfectly capable of 3 starring you, I'm not sure why you're complaining about getting hit by the higher player since you said you're farming and getting attacked by higher THs means you're losing less resources.
As a TH8, this will sure haunt the OP tonight.
TH7s can be as juicy too.
http://i63.tinypic.com/287lrbk.png
Sorry brother but this is the part of game and to me the best part of game. When ever a higher base attacks me with those mighty troops, I learn a thing or two. It polishes me as a player and I try new defensive strategies to protect my base. Moreover, you get to learn some attacking techniques. I hope you have learned by now how to do GOWIPES. It is totally fine for a higher base to attack lower bases. We all went through that phase and even if you are a strong TH9, you get smashed by some bully TH10. Not a big deal !
Honestly, this is how you can farm. I'm in the middle of TH10 and to be honest, if i'm looting 1 million a day and lose about 300k and a 12 hour shield/break, its a win win situation.
If only similar TH level can only attack each other, there wouldn't be much players with max TH10s. @Vexives, frankly speaking this is easy. Go to lower leagues where there are not much high level players.