Honestly, it Meta spun up a Mastodon site to host Meta employees and just have a corporate presence, the way they might have a Twitter account, that wouldn't be an issue at all.

The issue is that they're arriving as platform developers, not social participants. And that's their business.

We should be super suspicious of people showing up to sell the Fediverse, because you can't profit off of community. Community costs money, not generates it. To generate money, you need to exploit people, and exploitation is anti-social. Anti-community.