There are many, many models for forums and blogs. Some blogs have no comment function at all. Most require some form of registration for anyone wishing to participate or comments. Most have their boards wide open for read-only access, although some (like here) have special areas requiring registration.

For one writing forum, my price of admission to the higher levels was a writing sample to be judged by the moderators.

Registration is often wide open too, mostly serving to limit fakery and spammers, but a number of boards limit it. These boards often "open" registration for brief periods, then close it again. Policies for revoking privileges and banning vary immensely across the web.

Many forums and boards have sponsors or take advertising. Amazon is a common advertiser. The board is paid by Amazon for purchases made through their links (no ding to the purchaser), and many bloggers are quite open about it.

Frankly, I don't notice the ads for the most part, except those that do something annoying, like start a movie on me. This does not endear me to their product.