I think it's "corporate pragmatism" disguised as "altruism".
Like, Meta wants a mass of users on its platform very quickly. (The average person signing up for Threads will likely not understand federation and some of these people are not actually on Threads.) Meta also knows that these sorts of instances go down all the time - during the Mastodon migration in November, many instances were stood up - 25% went offline between then and now, according to this blog post. Some of that is server costs I bet. Meta can pay a small amount to instance owners to help offset those costs and also incentivize them monetarily to keep themselves online.
Much less altruistic, and this one is completely rumors and I'm afraid I've lost the original source which claimed it, but... Some chatter went on that Meta would let the instances it federates with choose whether they want to have the advertisements and sponsored posts federated to them. Obviously, if they take the ads they get more money, and that'll help offset more of the costs...
And on Meta's side, suddenly they can tell advertisers of this amazing built in user base they can sell ads to. Win-win, except for us (the users) who lose.