Yes . . . but . . . would the fact that PSA run mean you haven't violated the AdSense agreements by allowing AdSense to run on pages with nOkay topics?
Sub-question: Would the companies "behind" those PSAs be happy to know that ads promoting their good cause were appearing on pages discussing marijuana? Might someone report seeing "their ad" in an inappropriate context?
Just food for thought.
Related question: If you are going to run AdSense on a forum is it incumbent upon you, to state in your forum's TOS, that certain subjects are unacceptable and will be deleted? Since AdSense is contextual advertising is the forummaster's duty to see that the "content" conforms to AdSense terms?
Who knows? Anyone? Might there be a shift in enforcement of policy if quality becomes an issue?