Just as in the Justice system, the burden of proof is the on the side desiring a punishment.
It is not his job to prove it wasn't intentional. It is Riot's (and/or by extension the player reporting's) job to prove it was intentional.
If you let anyone claim "hey that guy was inting make him prove he wasn't" then you'd have a lot of players banned simply because they were too lazy or too tired of shit to argue they weren't inting - even if they weren't inting. As an example, and I don't like doing this, there was a 15 minute long 0/5/0 Nami game you played a while back where you die outside turret range multiple times, appear not to have finished your quest for wards on the item, bought refillable potion as a squishy support, bought boots of swiftness as your second item... I could pull out a lot of stats. Prove you weren't inting in that game.
Now, I don't actually want you to do that. I don't even think you were inting that game. But let's say your teammate did. Would you have to sit there proving you weren't inting? What if I said your explanation wasn't good enough? Would you feel comfortable getting banned for that game because you couldn't "prove it good enough"?