Your concern is legitimate, but you'd have to look at real data to see if the impact of that confusion is outweighed by the impact of forcing people to register for another payments service just to buy your product. I know that I don't want a PayPal account or Google Payments account. Without the option to use Amazon Payments, I would have not bought the Humble Indie Bundle. I suspect many people felt the same about the other payment options.
I seem to recall being able to make a payment with a credit-card via PayPal without setting up an account. That was at least a few years ago; is that no longer possible?
Probably, but part of the reason I didn't want to register for a new account was just so I wouldn't have to go through inputing all my information again.
