Why is that such a problem? We are no one to judge if a member decides to express their laughter. Like, there's a section for comments under youtube videos, no matter that people can just like or dislike the video
As the comment section is made for you to express yourself about the video (or even, anything else, really); + in here, "Hahaha" isn't a comment at all, and can be just done by liking a post, rather than commenting with 6 letters repeating itself.
If you actually go to a YouTube video's comments, people are either debating on something, or saying something nice towards the creator. Or just general trolls; something that cannot be done by just liking or disliking the video, for obvious reasons.
This is a forums, and not a YouTube video. Don't compare the 2 together, there's nothing that resemble them.