Yes, there is only one tag in the tag table, which is why your query worked in this case. However, it is incorrect for all instances. Calling the MAX function with a string would return the string with the highest alphabetic value. That is clearly not what we're looking for here. The following would work:

SELECT tags.tag  FROM tags INNER JOIN statuses_tags ON tags.id = statuses_tags.tag_id INNER JOIN statuses ON statuses_tags.status_id = statuses.id GROUP BY statuses_tags.tag_id ORDER BY COUNT(statuses_tags.tag_id) DESC LIMIT 1;