"arbitrarily detained"
To detain:
- keep (someone) from proceeding by holding them back or making claims on their attention.
- keep (someone) in official custody, typically for questioning about a crime or in a politically sensitive situation.
- officially seize and hold (goods).
None of the above are applicable to Mr Assange. He is not being held back by the Ecuadorians, or in official custody, or being seized or held.
In fact, being where he is is STOPPING him being in custody, which is kind of the problem.
Sorry, Jules, it's really time to come out because even the Ecuadorians are sick of you now.