The merchant is paying extra, and that cost is hidden in the price I pay.
If what you really meant is that the cost of the credit card transaction is implicitly in the price everyone is being charged, then sure, I see your point -- except that I think it actually strengthens the original poster's argument for using credit cards! If you pay $25 for a widget in cash and I pay $25 for the same widget on my credit card, then we're literally both paying the same price, but I'm getting the ostensible benefit of using the credit card and you're not. (I would actually argue, unlike the OP, that this is mostly true for debit cards as well, in that you still have a bank fighting to get your money back in a way that isn't true with cash.)
We also have an "EFTPOS" network that is independent of Visa/MC. It's a debit account network owned by the banks that give you access to savings or cheque accounts. It's charges are much lower than Visa/MC, so merchants like when you use it and don't apply a surcharge.