Sony owns and hosts MJ Vevo channel. If they start cleaning comments on MJ channel, then they should clean every singer or band channels too, and there are tons of singers and bands under Sony's label. I cannot see them spending money to clean nasty comments from nasty trolls from million if not billions of videos.
From Wiki:
Vevo (stylized vevo) is a video hosting service owned and operated by a joint venture of Universal Music Group (UMG), Google, Sony Music Entertainment (SME) and Abu Dhabi Media.[3] It launched on December 8, 2009.[4] The videos on Vevo are syndicated across the web,[5] with Google and Vevo sharing the advertising revenue.[6]
Vevo offers music videos from two of the "big three" major record labels, UMG and SME. EMI also licensed its library for Vevo shortly before launch; it was subsequently acquired by UMG in 2012.[7][8] Warner Music Group was initially reported to be considering hosting its content on the service,[9] but formed a rival alliance with MTV Networks.[10]