If a spammer wanted to see what a site took down, all they'd have to do would be to keep track of all the comments (including ones not by their accounts). Then they'd have some idea how the filter worked.

If you make the list open then honest people can see too.

The only reason you'd keep the list of what's spam private would be to maintain (perceived) infallibility. Every filter makes mistakes, only some filters let you see them.