This rhythmic pattern is something that inevitably formed from the characteristics of the Japanese language called 'kaionsetsu' (open syllable), and it should not be considered as a restriction or rule that exits solely for the haiku.
この韻律は開音節という日本語の特質から必然的に成立したリズムであって、俳句の制約とか、規則と考えるべきではない。