That’s why they have stealth enshittification techniques to disguise them.
Promotional posts don’t magically show up on Mastodon for users who don’t follow the brand account that posted them. That’s not how Mastodon works.
If I were to follow Marvel Comics via Mastodon, boosted a post, then my followers would see the post, just as they would see the post if I made a screenshot from a Marvel tweet and posted that to Mastodon but Marvel posts would not just show up for random Mastodon users just because Marvel paid Meta to promote a post on Threads. Only Threads website and app users would see such posts.
It’s amazing how uninformed Fediverse users are of the basics of how the Fediverse works.