After logging out of my account to test every amateur radio related forum I visit I have found that most of them allow me to read most boards if not all. The island, QRZ, QTH, Eham, Radioreference, AMfone, etc. However, most only have a general chat section if that when talking about non radio related material. IE most radio sites talk about only radio topics. Or rather, that is what is intended. Most of my major gaming forums allow me to read everything while not logged in as well. I also visit reptile forums and they run about 50/50 open to public reading vs closed. So with that group it is a toss up.
I cant speak for any other type of forum.