I upvote myself as well on my posts and comments on my on post. However I also make sure I upvote others as well, and the comment is usually relevant to the conversation. But I suppose you are right and it is his stake so he can choose to upvote what he wants.
Writing a vague or irrelevant short comment then upvoting yourself whilst not upvoting others leaves a bitter taste in the mouth for me. Like you said he could easily just write a post and do the same thing...