TLDR
He was raided for unrelated reasons to his mastodon server, the police have a seize it all policy.
There is talk about changing policy to exclude things like servers and similar devices that are not related to the initial reason for the search. There doesn’t seem to currently be laws or rules about what police can or can’t do with data.
That’s the whole article basically
Good TLDR
I would also add:
Data gathered from the raid can be used to investigate and prosecute crimes unrelated to the original seizure.
Mastodon does not currently encrypt direct messages.