It is easy to tell the difference between INTing and a bad game. Running from spawn to turret is pretty obvious.
It's not easy, unless you're using that aforementioned "run it down mid" scenario. And the issue is, you can't just punish obvious intentional feeding with a permaban. You have to punish all intentional feeding cases the same way - obvious or no.
And another issue is: Permaban from the gate means no room to reform. Nobody likes intentional feeders, but they still should get at least one chance to straighten out. You may not like the thought - and I know I particularly don't care for it - but Riot believes in at least one shot at reform.