So when a charge is made against a credit card, you have the option to do a "chargeback" - this is meant to be used for fraud. In this case, the argument is that Reddit fraudulently changed the terms of the program after people had already paid - being in "material breach" means they made a binding promise to provide a thing and they failed to do so. Chargebacks are really, really bad for a vendor. They lose the money, and they get a penalty fee, AND if it keeps happening the credit card processor can crank up their overall fees or even drop them as a bad customer.

EULAs are not legally binding and any court and credit card company on the planet would accept that you had a reasonable belief that you would be provided the services that were offered when you paid for Reddit Premium.