Actually, at least in the US you could be having legal problems... you're circumventing access controls, which may or may not be a "hacking crime". Also, by using the Twitter API you consent to its Terms and Conditions, so they have legal grounds to sue you (for example via trademark law; you most likely will advertise that your app supports Twitter login, but then Twitter can sue you as you advertise Twitter compatibility while violating their T&C).
It's IIRC never been tested in court so it remains an effective FUD, even without the Apple/Google store banhammer.