Well, I'm going to take a stab at this one. I am Jewish but I can't possibly speak for all Jews or most Jews or even all Jews of my own congregation. Just like there are many denominations of Christianity, there are many types of Jews with very different beliefs.
The idea that an atheist can still be considered Jewish but not a convert to Christianity has nothing to do with which is a "bigger" sin. The idea is that somone who professes no belief in God is simply a non-observant Jew. While someone who accepts Jesus has actively turned to a new religion. They have made a choice to exchange one religion for another. Now personally, I am not saying I agree with that view. I'm just trying to explain it. In my view, there are so many beliefs that have a place in Judiasm and I don't see why believing in Jesus woudl necessarily preclude someone from being Jewish. But I am not an "orthodox" Jew. I don't accept many many things that the Orthodox teach. The Orthodox (and many non orthodox) would tell you that the messiah was not meant to be devine, that the trinity is not monotheistic, that the Messaih was supposed to bring peace to Isreal, ect... Please don't ask me to defend those statements becuase I'm not equipped and I'm not Orthodox.
I will also say that some of the antipathy towards accepting Jews for Jesus comes from a history of fear of Christians forcibly converting Jews. Hundreds of years of persecution can make a people a little skiddish!
Now, as too why someone not born Jewish would want to accept Jesus AND take on all the Mosaic laws, I have no clue. Those laws were only meant to be applicable to someone born Jewish or converted to Judaism. Judaism NEVER taught that you must be Jewish to get to heaven. We always taught that the commandmants were an extra blessing given to the Jewish people. Non Jews would have a place in the Kingdom of God just by following the Noahide laws.
Sorry, If I'm rambling a little. Its late!