Contact is allowed in a marking contest if is incidental AND if the player is legitimately attempting to mark or spoil. Wilkie was obviously trying to spoil, but the contact across the shoulder was clearly more than incidental and is therefore a free kick.
Players can spoil from behind, but they can't make high contact whilst doing so (unless incidental). It's not hard to understand. This is not some new rule that's suddenly been introduced either.
Is there an argument that Wallis infringed himself? Maybe, at a stretch. But it came after the first infringement.
The silly thing is that a player can run, jump and knee a player in the back of the head spoiling its play on.
But brush past the shoulder with a hand or arm its a free?
That is quite difficult to explain to a non AFL follower watching the game for the first time. Hell, its hard to work out for veteran footy fans.