Twitter talked about its platform approach and long-term strategy, "As our primary concern is "long-term health and value of network," we'll continue to forgo near-term revenue opportunities in the service of carefully metering the impact of Promoted Tweets on user experience. For this reason, aside from Promoted Tweets, we'll not allow any 3rd-party to inject paid tweets into a timeline on any service that leverages Twitter API". Twitter reasons for prohibiting these ads:
First, 3rd-party ad networks aren't necessarily looking to preserve unique user experience Twitter has created.
Second, basis for building lasting advertising network that benefits users should be innovation, not near-term monetization. We believe it's our responsibility to encourage creative product development and to curb practices that compromise innovation.
It's important to keep in mind that Twitter bears all the costs of maintaining the network, protecting Tweet stream against spam, supporting user requests, and scaling service. Indeed, Twitter will bear many of support costs associated with any 3rd-party paid Tweets, as Twitter receives support emails related to anything a user sees in a tweet stream. The 3rd-party bears few of these costs by comparison.