The answer for this is pretty simple -- you want promoted tweets to show as promoted, and old clients don't support that type of rendering. There are hundreds

The answer for this is pretty simple -- you want promoted tweets to show as promoted, and old clients don't support that type of rendering. There are hundreds of twitter clients out there, many of them written once and updated sparingly. These clients and apps aren't checking for the "isPromoted" field when they get a JSON blob back from the server -- they just dump the text / author / profile pic into whatever rendering engine they have. So a promoted tweet shows up identical to a read tweet. Even worse, I doubt the developers are going to waste their time adding support for promoted tweets if they don't get a cut of the ad revenue. And lastly, Twitter also needs to accurately count impressions on promoted tweets. on the web they can do this with some degree of accuracy -- on clients, it's trivial to filter by the isPromoted field, resulting in delivered tweets that aren't displayed and angry advertisers.