I don't see why it would be a problem. For a start, if Oli did spend time developing the blog on his theme store, he would probably be linking to other theme stores less because he would be doing less news posts and more tutorial posts. Even if he continued to actively promote WordPress themes and plugins from other developers (e.g. through theme and plugin lists), why would any theme or plugin store have a problem with him doing so? As long as he sends them traffic and customers, the theme and plugin companies will be happy.