Apple isn't required to help you promote your content if they don't want to, but they are also not allowed to block your access to legal content, regardless of what they may think of it.
What law forbids this?
What I wouldn't like is Apple parsing the podcast feeds I enter into iTunes and potentially block me from adding certain feeds. That should be non of their business.
I'm not sure if a private company is allowed to block their customers from accessing content that the company, in this case Apple, simply dislike. That's may be perfectly legal.
Would Apple be allowed to block users from accessing Google from an iPhone, if they wanted to? They can't if they do to promote their on alternative, but can they if they just feel like it and don't benefit?