And I agree, I don't think the UAE cares about Portland. They might care that US security services had taken an interest in this individual though. Are you saying that the UAE security services are totally subordinate to the US and just act as puppets when it comes to US citizens? I would have thought they'd have a bit more independence than that, even if they do cooperate at some level.
It is possible that they took an interest in him out of the blue, but he had done nothing suspicious in UAE to warrant such brutality. Suppose he had done something suspicious in Dubai, does it seem likely that any client regime would do this to a US Citizen to ask them "who was the bearded man you met with last Thursday?" The story that he was asked about the mosque in Portland is much more plausible.
Sadly, the US exercises enormous influence on small regimes like the one in UAE.
It seems far fetched to think that the UAE would take a sudden interest in the security of Portland, Oregon but perhaps that's possible. What IS impossible is that a country that's a client of the US would dare to hold, let alone torture a US citizen for a 106 days without the explicit approval of the US.
You think the guy's story is far fetched because he just happened to be in the UAE. Not so much, really. They keep track of the movements of many people (1.5m according to Citizenfour) so they would have known that he was in the UAE. At that point it would only have required one bright bulb at the FBI to come with the idea "hey, why not we get our buddies in the UAE to have a chat with him?"