The main issue with To Kill a Mockingbird and the reason it is being removed is its white saviour narrative. A book about racism, written by a white woman, and the hero and moral centre of the novel is? A white man.
Black people in it are firmly in the background. Front and centre are white folks. Blacks are helpless and dependent. It's up to whitey to save them.
This toxic, narcissistic trash should not be polluting the minds of any child.
A major problem is america has never owned its racist foundation. It makes excuses galore about the 'founders' (the elite planter class who dehumanized the Natives as "savages" in the white settler document the 'declaration', and having ethnically cleansed the Natives from their fertile land and forced the few who remained into reservations, owned hundreds of Africans to work the stolen land).
https://www.theguardian.com/world/2018/jul/05/facebook-declaration-of-independence-hate-speech
So instead of owning it, as Germany has done with their history in WWII (excellent book on this which contrasts the two countries), and shown some humility over your crimes, they look to minimise it and write books about how moral the white man is, a result of which is the white saviour trope which To Kill a Mockingbird is a prime example, thus is aptly "their beloved book". Of course it is (the white folks anyway, for black people it is a demoralizing book), as it allows you to excape the reality of what you have done.
To quote Hitler in 1928, you "gunned down the millions of Redskins to a few hundred thousand, and now keep the modest remnant under observation in a cage".