@KitKat Cross-posting is fine, automatic mirroring is not because it grabs things out of context and many of them are Twitter-specific.
I was complaining because of a retweet of a picture from a locked Twitter account, no context was provided nor could be found.
As someone pointed out it would be nice if Twitter provided a proper gateway to the Fediverse that would allow actual interaction instead of just getting a copy. It's quite unlikely that this will ever happen, though.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!