What would be a valid reason to allow this? That just seems mind-numbingly stupid.
I use this feature all the time. Users get Markdown wrong, give titles to issues that don't make any sense, have typos, etc. Being able to edit issues helps me keep the issue tracker easier to understand and navigate for maintainers and users.
Every feature can be used. That doesn't mean every feature should not exist. The fact that the edit history is still visible means it's next to impossible to abuse the feature. It works fine.
If you actually use GitHub as a social network of sorts, there are many reasons to do edit comments. All the edits are visible anyway. You're on Git-Hub, you can already edit everything you have write access to.
Either way I think it's a pretty stupid feature the way it's implemented; it should show the edit more clearly or indicate that the comment has been written by multiple people (like StackOverflow does), especially if edits change more than e.g. 10% of the original comment.