Any legal issues with using Twitter's API (posting, reading) when you build a twitter client? do you have to be "licensed" by twitter for this? do they impose limitations on you to keep their client's in an advantageous position?
Your access is controlled through the application tokens.
Although Twitter has the right to cut off your application by revoking permissions without publishing the reasons, I believe they won't go that route as long as you're playing by the rules defined in their TOS.
The official Twitter client uses the same API as any other dependent application.