I'm fairly new, too, so take my word as such...
But I think private channels are "unofficial" according to Roku - they are developed by private programmers/developers that have taken it upon themselves to make the channel available to the Roku viewers. Don't worry - it is easy to figure out whether a channel will have fees associated with it once you get in and try using it.
technically, a "private" channel is a means of publishing a channel for widespread use to Roku's servers enabling users to add it to their box. This is a way for developers to distribute and test their channel operation (and have other users test it as well) in a real-word situation, rather than it residing solely on a developer's own box. This process requires NO approval from Roku, so the content within said channel might not be from official sources (official meaning the owner of the content.) The private channel process is also used by private developer's to bring added value and content choices to the user community without it needing to conform to Roku's terms of service. That being said, if a complaint is issued to Roku over content violations in a private channel, or it blatantly comes into conflict with either their or an official partner's channel unlawfully, it is the discretion of Roku to remove it from their servers. That is what the situation is with the afore mentioned channel. Long story short, be cautious when adding a private channel that requires payment.. Many times it is a legitamate source where the publisher/creator is asking for a nominal contribution to aide them in their continued development, BUT if it is a "questionable" service and something goes wrong, there is no fault on the part of Roku or recourse with them because of.
The open development and the private channel process is one of, if not the greatest strength of Roku. Add lots of private channels, because that's where you'll really see added value of the Roku.