American Citizenship. We (as a whole of American citizenry) have no problem accepting that a person born to American citizens is an American. We also accept that the constitution and the laws of the land define what it means to be a good citizen. And we accept that not just anyone can become a citizen. They need to live here, agree to obey the laws, learn about the US, pass a test, take an oath.
If you and I decided that someone from another country should be a citizen, we can't just hand them a flag and say they are now a citizen. Even though you and I might think that person is a wonderful person and will make incredible contributions to the USA, they cannot become a citizen outside of the law.
Similarly, in Judaism, Torah is our "constitution" and our body of law. The difference is that it was not authored by man, but by G-d. We do not have the right to change it. We DO have the right to apply its teachings according to a given set of rules.
There are lots of Jews who do not abide by the body of laws given by G-d but they are still considered "Jews," just as there are lots of Americans who do not abide by the laws of the land are still considered "Americans."