Because I support open source technologies, and support the idea of the fediverse. I like to be able to get content all in one space, or my friends being able to follow me without having to use the same platform. corporations hosting would be less than ideal, but if they help invest money into the fediverse and help it grow, I'm all for it. I can always leave their server if I don't want on it, and go back to how platforms like Mastodon or Lemmy are today. Either it stays how it is today, or we have a chance of hitting mainstream appeal and letting more people see it's benefits.
Like I hate meta just like alot of other people here, but I know Tumblr talked about adopting activityPub. Are they are "good company" or are all companies bad? What about valve supporting Linux gaming, they obviously benefit from helping it, but Linux gamers have gained a lot, are those actions bad as well? My point is, are you against them solely because they're "corporate social media" or are you against their practices in the past? I understand if it's the second one, because Facebook, Twitter, TikTok, are KNOWN to show content to keep people scrolling and interacting, to get ad money, for better or for worse. But The fediverse doesn't necessarily have that problem, especially with mastodon and lemmy. Thread may show ads to their users, but Lemmy and mastodon users will probably never get those. Basically federation with meta just provides more users/content to the platforms, not forcing everyone to switch to thread.
Also if you're just mad at the fact that Facebook is a company, Mastodon is owned by a (non-profit) company. A company is just made up of people, and it isn't hard for once community focused companies (like Linux distros like Ubuntu or redhat) to get corrupted over time, and defederating with meta ISNT going to prevent that
To your edit, most people aren't "defending meta" they're defending just not going scorched Earth before we have the facts, defederation can happen any day, why do it before we even see how it will be? I see no reason to exclude ourselves from millions of other users yet, yes meta is very concerning and should not be trusted, but what are we even trusting them with yet?? If they federate, nothing really changes to them, if we defederate, nothing changes for them.
My point is people are allowed to have different opinions than you, and one could say to you "why is every one of your comment history bashing expanding the fediverse". Just give it some time and see how it goes and we react from there, ruud and his team are just as much users of the fediverse as you, and will do their best when the time comes.