> For all we know, this detention is completely unrelated to WannaCry.
No everyone has already determined 'wow he did a good deed' and 'us law enforcement bad'.
The fact is he is linked to this event and a person of interest who they want to get more info from. As such it makes total sense they would detain him for some questioning searches and so on.
If you are someone who stops a crime you will also get questioned by the police. For all they know you are covering your own tracks and had a role in the crime. This is almost a cliche in movies and tv.