I seriously doubt that if the Boy Who Lived killed a dementor in front of the Wizengamot, their response would be "let's let a dementor eat him to make an example of him." They may not respect him, but these are still people who've been celebrating Harry Potter Day for years.

Just because the members of the Wizengamot are stuck in a hate-spiral with respect to Hermione doesn't mean that they're exceptionally evil people regarding other issues (this is as good a place as any to take note of the chapter title.)