No it is not normal to get logged off after you have read one thread.
Sometimes I can read two or even three threads without getting logged off, sometimes I'm even logged off when I click "first unread posting". (that is, I select "unread postings", select one thread and get the first page of the thread displayed. I select (top right) the "first unread posting" and instead of jumping to the first unread posting the page stays the same, only "first unread posting" disappears (meaning you are not logged in) and in the top right I see "login" instead of my username. Sometimes in the same situation I get the login screen instead.)
And I know, half a year ago, I never was logged off unintentionally reading even 10 unread treads.
Seems maybe some people are only affected for where they are located then.
I agree. Maybe it somehow depends on the provider.