I don't know which is worse--- slavers or people who try to tell
other people what ought not be sung.

But seriously, 'Amazing Grace' is a song that has been validated
by its singers, and the times at which they've sung it. So very
often it's been sung at particularly poignant times in the lives
of individuals and groups to express the sentiment that there's
something bigger than all of us which binds the universe together.

It's the singer--- not the author--- and the times and company
in which it's sung that bespeaks the merit of a song.