He's not - there is a reason for that instruction.
Many people think that mark a solution is a way of "closing the question/thread", but that really is not what it is intended for.
When you mark a post as the solution, it really serves two purposes:
1. It indicates that the question has been solved.
2. It also updates the thread so that the solution will now appear right under the original question (open any post with a marked solution and you can see this). That way, it is easy for anyone who may be researching these questions to easily see the original question and ultimate solution in a single glance (and not have to hunt through a thread that may have dozens of posts to find the solution). In that respect, it is not only important to mark a post as the solution, but also mark the correct post as a solution (people often mark their own post acknowledging that it has been solved, but not the actual post containing the solution).
Sometimes people will say that they have solved it on their own, but did not post the solution back to their thread. If they post their solution to the thread, they are then welcome to mark that post as the solution (as it actually contains the solution). But a post that does not contain the solution to the problem should never be marked as the solution.
Hope that clears up any misunderstanding.
