Lawsuit on basis of what? IANAL, but I've heard many times that ToSes/EULAs which forbid you from querying their servers and reverse engineering their apps aren't legally binding in most jurisdictions.
A lawsuit would likely be based on copyright/database rights infringement if you are accessing a data source which the company only makes available under specific conditions which you are bypassing.
Craigslist v Padmapper/3-Taps in the US is a slightly analogous case albeit with 3-Taps scraping rather than bypassing restrictions on an API.
But - unless the data's included with the app - is that app author's responsibility? That sounds like suing Bram Cohen for illegal downloads over BitTorrent.
BitTorrent is completely neutral and just a mechanism for distributing content which can be contrasted with the others. Anywhere where there is a deliberate attempt to bypass restrictions on the availability of data then you may get into trouble. Even more so when you bypass then attempt to monetise like with 3Taps...
