To some extent they do, and as I mentioned it's both a matter of practicality and morality. There's obviously a problem with saying that people who are or may be persecuted as part of a group, are excluded from help by that group if they don't belong due to some technical definition.
According to the Wikipedia article on Israel's Law of Return, in 1970, it was extended to people with one Jewish grandparent or married to a Jew.
"There are several explanations for the decision to be so inclusive. One is that as the Nuremberg Laws did not use a halakhic definition in its definition of "Who is a Jew", the Law of Return definition for citizenship eligibility is not halakhic either. Another explanation is the 1968 wave of immigration from Poland, following an antisemitic campaign by the government. These immigrants were very assimilated and had many non-Jewish family members."
