Mozilla has to make statistical assumptions about their users, and statistically any random person's DNS will be set to resolve using their ISP. Mozilla may see this as predatory (ISPs in general are not exactly great companies who care for their customers), and so from their perspective comparing "Average DNS use" to "We vetted a company & trust them & we feel good making this decision for our users" sounds worth doing. There may be some latent assumptions about "well power users will just turn this off like they do everything we build" which would also support the action taken.
With that defense in mind, it's still a shitty thing to do to your users. I don't like it, but 99% of the world doesn't even understand what the change means.