See, I thought it worked like this-
All are infected with the 're-animation' virus. You die in any way that leaves the brain intact, then you come back & turn into a walker.
If you are bitten or scratched by a walker that has already turned, then you get a feverfrom the walker's bacteria or something that eventually causes death, therefore, you turn.
Before it was revealed that all are infected, folks thought it was the attack that caused the walkers, but now they know it's just the fever that leads to death.
So by logic, if any walker contact that entered the blood stream will cause the fever, then why wouldn't contact with an open wound cause the same fever?
I guess in retrospect, almost all the rules the characters have established have in one way or another been proven to be maybe based on circumstances.
It was said that the walkers aren't interested in those that are alredy dead, but one ate Lori, & there was the one that Andrea & Daryl came across in the woods last season. He had hung himself, but the walkers had eaten his legs.
Then there's T-Dog. It looked to me like he had been gutted, but I really didn't see much damage to the skull area. He had much less damage than the bicycle girl from the first episode, but we got no indication that he was turning. Even after the guys found him, they made no attempt to destroy the brain.
Maybe that's the point. Just like it'd happen in a real situation where survivors are cut off from communicating with others, you operate on assumptions based on your experiences, but you never really have all the facts or a complete picture of the event.