Whatever the reasons may be, or even if Twitter is "on the right side of the law" I would rather see them embrace this entrepreneurship than go the route of the bigger tech companies and squash any bit of choice. Typically people use third party add-ons because they enhance the experience. What would twitter be like if they banned all the third party services that convert longer messages and post a shortened version on twitter? I can imagine a similar scenario if, for example, reddit decided all of a sudden to threaten the creator of "reddit enhancement suite" or if they decided to shutdown Imgur, which I think that symbiotic relationship was already mentioned here.

Time to look for alternatives.