Not necessarily true of course, but you could at least assume that your users have given you a license at the moment they posted it on your forum, so it would be their problem and not yours. So your use case for this is legitimate, I’m 100% convinced of that.
But no sane open source project would risk getting ex parte taken down because of some legal argument, even if they are / turn out to be right and the complainer is not. So I think the import script is the right place for this.