This isn't anything new - http://wank.wordpress.com/
The reason they don't allow commercial blogs (it's not just limited to PPP, they don't allow AdSense, affiliate marketing, and a variety of other activities which could be construed as commercial) because they're trying to keep it splog-free.
It's also the number #1 reason why I never recommend someone start their business using a third-party service/domain. Always host your own domain/website and don't use those services for anything more than satellite or personal blogging.
Abide by the rules (which they have every right to set...it's their service) and you should be fine. If you have questions or a problem, then feel free to submit a support request or post in the support forums.
~ Teli