The difference between Open Source and Free software (incidentally, I am in favour of the latter, but this is irrelevant) has to do with ethos and values, not copyleft.
From https://www.gnu.org/philosophy/free-open-overlap.en.html:
"Among all programs that are open source, only a minuscule fraction are not free."
From https://www.gnu.org/philosophy/open-source-misses-the-point....:
"The two now describe almost the same category of software, but they stand for views based on fundamentally different values. For the free software movement, free software is an ethical imperative, essential respect for the users' freedom. By contrast, the philosophy of open source considers issues in terms of how to make software “better”—in a practical sense only. It says that nonfree software is an inferior solution to the practical problem at hand."