_What's wrong with powerfarming? _
If we're talking about the same type of powerfarming, It's uninteractive. If you do it successfully, it's essentially you vs creeps the entire game and if you fail you've starved your team of a team member and forced a 4v5. It doesn't feel good on any side.
Stayed in lane opponent did the same?
Many of the times the bounties appear is when a laning opponent roams more than another laning opponent. Very rarely does it occur when your laning opponent is in the same lane as you. If this does happen and you are "better than him" the bounty should mean nothing. If you're ahead in gold and stronger than your opponent, you shouldn't be dying in the first place. That would mean either A) you lost to an opponent that was weaker than you both in gold difference and someone that you're "better than" or you overextended, you should be penalized. This also helps turn the game more team reliant instead of one person being a bad laner and affecting the entire team. It still happens, but this system suppresses a little of that. Your mid laner is incompetent and can't cs? He keeps dying? That's fine, someone else, hopefully someone good on your team will be able to pick that bounty and extra gold off of the enemy.
"Their fed laner gets even more fed"
"this actually makes it worse for a team that is behind in some lanes. It makes it harder to come back"
Riot has put this system in place to both make it easier to come back from losing games and easier to close out decided games instead of waiting for the baron dance. If you're successfully able to grab the bounties while you're behind, you get that extra gold to spring you back forward instead of having to kill the enemy as many times as he's killed your own laner. If the enemy team stays consistent, then they're able to close the game faster instead of being in a state of limbo where they have to wait for baron to spawn to really do anything. This wasn't just put in place for just comebacks. They're giving more options for both sides of the situation.
Also many of the bounties that are given in game happen to just be 8 cs bounties. And the bounty hunter doesn't kick in until you're killing your opponent in cs anyways. It'd be wise to read the actual patch notes of these changes instead of just referring to me, but I'll be here to inform as long as you're willing to listen to me.