Most of the forums have restrictions on new members like they are not allowed to use message feature and can post less frequently. I do not think that now allowing them to open a thread will be right. First of all, there may be cases when a members goes through some forum post through search engine traffic and then joins here to ask doubts. If you do not allow new members to make thread here, how will such a member ask his doubt. Another thing is that thread is something already public and thus if someone is misusing it, readers can use report to moderator option.