This happens when you accept something when you didn't prove it to be a fact. The funny part is that they are now believing in something (there is no god) which they again didn't prove, soon they will have doubt and uneasiness in their life about this also. If they would have accepted Islam based on facts, or their parents taught them what Islam says (What Quran says) leading them to understanding Islam, there would have been no rational evidence based reasoning to leave Islam.
They are no different from my all my Christian friends who are only called Christians because their parents were Christians. This is a common problem for the majority of the Western families, unfortunately Muslim families in the west are doing the same, they don't discuss the Quran, they don't teach their kids about the Quran, and they are expecting their kids to follow the right path w/o any evidence.