In the comparison matrix Premium says Music and Music Videos Ad Free, while Lite doesn't have that checkbox so it looks like it's just music videos that'll have ads.
"For Premium Lite, ads may appear on music content, Shorts, and when you search or browse"
(although I'm wondering if it means the ads between video thumbnails...)
Singles used to be an advertisement, they would make the artist more known and be ads for the full album. This is no longer the case since the whole album is available on youtube on release day. Public tv has shows you can watch for free, you have to sit through commercials though, why would it be different for music ? Producing an album is extremely expensive and we don't expect movie studios to distribute their movies for free because they'll sell merch. Not even children movies that are obviously made to sell toys.