In the case of atheism/unbelief the same analysis as that above could apply- but there is an additional factor. There is a concept called "Tinok she lo b'nishma" "a baby without understanding". What this principle means that there are times that we do not hold a person liable for a transgression since they do not have the understanding/education/background necessary to not commit that particular sin. In the modern era, the vast majority of atheists within Judaism fall into this category. The same thing would apply to non-Jews as above- did they ever have the chance to do other than what they did?

Why the more lenient approach to atheists and non-belief? Here we need to look at the implications- in the case of believing in polytheism we are denying G-d but accepting other divinites, in the second instance we are denying all divinities! The first case is far worse- it claims that divine power does not exist and then abrogates G-d in taking up polytheistic beliefs; on the other hand, atheists deny all divinity and thus do not promote some other divintiy over G-d
References :
Orthodox Jew; Iggeros Moshe

So we don't tell others, non Jews, not to be Polytheistic, or Hindu, or Catholic, etc. We simply state, logically, that no Jew can follow another faith AND remain Jewish.

Judaism says that ALL humans are EQUALLY the children of G-d.

Also, a person who is born Jewish can be an Atheist, as they are not violating the commandment against worshipping 'other gods'. They remain part of the Jewish family. A person is an EX member of the Jewish family ONLY IF they adopt ANY OTHER FAITH.

Hope this helps to answer
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