There are 2 things to consider here imho:
REST api are considered to be consumed by programs. Those xml users are mostly developers; and for them we have the docs available to find how to specify the request for the timezone to be set/get. I think it is transparent enough even if they will have to deal with regions (continent) and timezones (country/state) as two separate entities.

We would need 2 controllers (or at least 2 kinds of routes) for separate handling of web-ui and rest requests, which was the case before I refactored it into one. Which I think was in-optimal. I think it's not clever to sacrifice less code and single responsibility principle for the reasons you specified. But in the end it's a matter of discussion which solution we do prefer and maintain.