Haiku is a Japanese word and in Japanese it remains the same in the plural. It's tempting to judge people's commitment to the form by whether they refer to two haiku or two haikus. I always refer to haiku as haiku in the plural and haikus as a plural annoys me, but as someone who speaks Italian, these also annoy me, yet are accepted without comment:
two pizzas - no! - the plural of pizza is pizze!
two paninis - no! - panini is already plural, panino is the singular!
two cappuccinos - no! the plural of cappuccino is cappuccini!
You get the idea. So why do some poets, who happily probably drink several cappuccinos every week and probably at least a couple of paninis a month, get so upset about anglicising the plural of haiku?