Since the Public Mobile forums are a necessity for members to troubleshoot and resolve issues with service, I propose that it should be free to access via the PM network. In other words, it shouldn't cost me data to view any of the PM websites, especially the forum and self serve portions.
I'm not sure about implementation, but I recall that Virgin mobile did a similar thing when accessing your account via your phone, it did not cost any data to do so (two years ago when I was with them).