Reguardless of what champions were on my team or the opposing team, as long as the losing team isn't totally incompetent they are able to turtle in the fountain and farm as long as they'd like. I'm not saying I have never done the same thing, as a matter of fact, when I am on the losing team I absolutely fall back to my nexus turret and poke 'til the cows come home. That doesn't change that fact that it needs to be fixed.
The ability to turtle on TT wasn't a mechanic purposely introduced to balance champions with an early game advantage, it was a lack of foresight on the part of the developers. Almost every game of TT with decent players on each side comes down to one final teamfight after every champion has their full build, and it's absolutely ridiculous. It makes for stale gameplay and unsatisfying victories, reguardless of who wins.
And as for those of you who are ignorantly saying "Oh, if we can't turtle then OP champs will own 2 hard", adding a draft mode for Twisted Treeline would solve that problem as well.