epoch timestamps don't need timezone offsets. The epoch timestamp is the number of seconds from a specified "zero" time. That number can be translated to a time in any timezone. E.g. right now:
% date +"%s"; date; date -u
1408580844
Wed Aug 20 17:27:24 PDT 2014
Thu Aug 21 00:27:24 UTC 2014
Sure, an epoch timestamp can be translated to any time zone, but what was the original time zone?
Consider these questions: Do I use Uber more often before or after noon? Is this a photo of a sunrise or a of a sunset? How late do I typically go to bed?
In the case of Uber we could try to guess the time zone from the coordinates, but it's not trivial.
The API appears to be well thought out--except for the epoch timestamps without time zone offsets :-(