While it's true that Safari, Youtube, and even Mail can be used to access unsavory material the difference is these apps can be restricted by parents, individual third-party apps cannot.
Aside from having parents block all third-party apps, which neither Apple nor developers want, the only option is to have a "your kid could potentially access adult content" rating.
Everything else in this article is fair comment though. If Microsoft/RIM/Palm get their act together I can easily see many developers defect.