First, I think we have understand that there are differences between European and American credit and debit cards. There is something called an interchange fee. This is the fee that the bank collects from the vendor when handling the transaction. Some website claim that, in this country, it is only 2%, but banks give you 2% back for using the card and I'm sure they are not working for free, so the interchange fee has to be more than 2%. I've heard it's more like 4% and I'm inclined to believe it. If a business gives a 4% cut to the bank for half their transactions, you have to believe that they have price that are at least 2% higher than places that don't accept cards.
The EU has limited the interchange fee at 2/10% for debit cards and 3/10% for credit cards. That is for cards issued by EU banks. They have allowed banks from outside the EU to negotiate their own fees with vendors. Maybe that is why you see places taking EU debit cards (2/10%), but not US cards. On my last trip (2017) of 20 nights, 2 properties where I stayed for 4 nights told me in advance that they only accepted cash. I only paid cash the rest of the time, so I have no idea what other places did.
In my 5+ months of German travel in the last 20 years, I have regularly stayed in small, "mom and pop" establishments, Privatzimmer, etc because I enjoy the intimacy they provide, but I have also noticed that I pay far less than the I would at the ritzier places. I attribute part of that to the fact that these simple places do not have to pay CC fees.
So go ahead and feel smug that you have been able to use your CC to pay for you room (and probably gotten miles for it), but rest assured, you have paid for it in higher prices.