Well it does seem like a spreading practice.
Personally, I really don't care about them offering this on single player games as long a I can still earn the items through playing the game if I wish.
In effect, the micro transactions are paying to cheat, so anyone that wants to skip through a game can pay for it
Now, if any of the launch titles required you to pay to win via micro transactions or made the grinding so difficult that you had to pay, I would have a problem with it. So far, no reviews for any of the launch titles have made the claim that was true, so I'm not really worried.
With games like GT6 on the ps3 gaining micro transactions, its clear that this next gen will also adopt such practices. KZ doesn't adopt those practices, so that is a sign that not all games will go down that path.
Killer Instinct doesn't adopt a pay to win micro transaction option either. You can buy the whole game, or you can get the free demo and just buy a couple characters you really want. They have taken a much better approach.