Secondary boycotts in law typically refers to union related actions, but you could draw parallels to how payment processors are putting pressure on the internet to effectively ban erotic art.

"For example, suppose you have a labor dispute with a dairy. The dairy sells its products to a grocery store, which sells them to the public. You may picket the grocery store to discourage its customers from buying the struck dairy products." - National Labor Relations Board

In credit card companies version, they put pressure on banks who work with them via credit card company rules, and those banks then refuse to do business with companies until they change their policies to suit the card companies. It's like boycotting, but instead of you choosing to avoid paying for a product in protest, the card company makes you unable to pay for anything, same outcome in that the company isn't paid until they change what they do or sell. Only difference is that it's not you choosing what to do with your money, it's Mastercard, Visa, and Paypal deciding what you do with your money.

And the problem is that most alternatives still have to go through Mastercard and others payment processing, and even if you work around this further and buy Steam's prepaid cards, some banks are on the credit card companies side and will make this harder. Worst case scenario, people have to resort to physical currency to get around these secondary boycotts. What percentage of the population who uses credit cards would be willing to go that far?

The only real solution here is for the law to limit the powers of the people who facilitate payments, so they can't use your money to interfere with what's available for you to buy.