In theory you’re already paying the merchant fee in the “price”. So merchant found a way to improve margins and credit card companies found a new revenue source
Or phrased less inflammatory manner: "Corporations can enter into contracts and engage in legal action just like people can". Even the much maligned Citizens United v. FEC basically boils down to "groups of people (corporations or labor unions) don't lose first amendment protections just because they decided to group up".
Except not everyone in a corporation has the right to speech. I'm prohibited by my employer to say anything on the company's behalf, but the C-suite and board are able to speak on my behalf. So, the company's leadership has a right to free speech, I don't.