I want to suggest that the real motive of these atheists is to attack Christianity and to impose their religion of atheism. If an atheist is going to put up a monument, it really should say something that reflects their worldview, such as:
"Do whatever you want to do, because you’re just an evolved animal. You are your own god. Life is meaningless; when you die you won’t know you ever existed. Become an atheist and live a meaningless life, and then die and never know you were ever alive!"
You know—this is really the message of much of the public education system today where students are taught as fact that life (and the universe) evolved by natural processes (naturalism is atheism). It’s no wonder kids turn to drugs, sex, suicide, and so on.
The message of Christianity is so different from the hopelessness atheism promotes. God’s Word teaches that man has—was created with—purpose. The life, death, and Resurrection of Jesus Christ has made possible the free gift of eternal life with God for those who receive it, by faith alone, in Christ alone:
"For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek." (Romans 1:16)
"For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast." (Ephesians 2:8–9)