They redirect you if you aren't logged in so you can't use them as an anonymous proxy. If you're logged in on your homeserver, you'll get that server's view of the post.
You can't post (because you're not logged in), so there's no issues with moderation. The toot is already federated publically, so there's no issues with unintentional read access. It doesn't need to contact the original server, so there shouldn't be any load/DDoS issues. I must be missing something…