Do you possibly just have no idea that deep packet inspection has many legitimate uses, and is critical to security in many networks? Or are you simply ignoring that fact for the sake of your argument?
I can help with some real world examples. One is Blue Coat.
This is much different than (my own wording): "anyone working in DPI for a company they know is selling their products to a police state".
It is absurd to blame open-source developers, researchers, or even employees at company's whose software has a legitimate purpose but is illegally exported and misused. They're just doing their job, since the technology has legitimate uses, as you've acknowledged. Blame the governments, not the programmers.
As a quick example, one strategy (although personally I've always questioned it's viability, but it's just one of many examples) is a network admin may install a filter that deep searches packets for common SQL injection or XSS strings. This is done as a secondary measure to possibly prevent malicious requests.
Other examples are if you want to force employees to not be able to send certain documents or information outside of the company for compliance reasons, you can scrub traffic for that information. Obviously more complex.
The general concept is that it's useful for when you know you do not want specific traffic crossing your network. Ironically, it's the same use case scenario with draconian governments preventing encryption, but in the production or corporate scenario the use case is not ethically unsound.