About Polygamy in Israel. 2 issues (broader as well).
1) If a Jew get's more than 1 wife he is arrested, when Arabs (in the Negev) do it they do not, and in fact get paid more money as a result. So the laws are not fair to begin with.
2) Why is Dina DeMalchut Dina applied to Israel. This is a Halacha for Jews outside the land of Israel under gentile Kingdom's (or rulership). Who says that it applies to Israel, to government that is Jewish and is secular on top of that?
Would it come down to a question of power and not authority.
+ for Askenasim this ruling is over as 1,000 years passed (assuming that it was in affect for 1,000 years which it wasn't really, but still).