The "reasons" for paying for OSS software are numerous and the point is not that people will always publish the source code for their N900 applications - the point is that source code availability is irrelevant to this discussion.
People do pay for OSS applications and the limited ability to make software for the N900 and get paid for it by people who want to pay for it will hurt the platform. People will just go to other competing platforms and those applications and users will be there instead of here.
Nokia needs to figure out what the heck it's doing. As I wrote in another thread Nokia has all the appearances of an organisation in civil war. Different people are obviously trying to push it in different directions and what we end up is a compromise that satisfies nobody.