I don't think the scratches cause the infection but the open area being contaminated by the walker is likely introducing the infection that kills into the bloodstream.
So if you have a scratch of any kind walker or non walker, and you get walker blood on it, I'm thinking it should make you turn just because it gets into the bloodstream. That's why the amputation if done quickly enough is supposed to help. As long as the infection chain isn't allowed to keep going through the body they shouldn't turn. (until they die)