I guess I'd call myself a "spiritual anti-deist", but I'll answer anyway
I do believe in the strong separation of church and state, and I can empathize with any non-religious group that doesn't want any of their tax dollars going to support religious groups. So I can imagine this monument being some sort of reasonable protest.
As far as "belief" goes, I can say more myself that what I believe in is the "Enlightenment". If my belief is a common one for atheists to hold then perhaps it's not accurate to say that atheists believe in nothing?