It doesn't make sense that the law exists at all. In a democracy let people have access to information (in this case the New Testament) and let them decide for themselves what to believe. The best defense against the spread of Christianity among Israeli youth would be for the Jewish leaders to actually refute the claims made by Christians. Or maybe their inability to refute the Christian message is reason for the law?
The reason for a Jewish state is to provide a location where Jews can live in peace. Many of those who move to Israel do so because they find themselves unable to live as Jews in their country of origin.
It is sad that these people move to Israel, let down their guard and then are induced by missionaries operating under false pretenses to abandon the faith of their ancestors. It is doubly sad since in many cases these people's parents recognized that their children were becoming alienated from their faith, and the parents decided to move to Israel to revitalize their connection to the Jewish people.
I don't know why you think that missionaries have a perfect right to approach these people in Israel, a foreign country, and trick them into believing that they can accept Christianity and continue to be true to their Judaism.
Knowledgeable Jews have no problems refuting them. The problem is most Israelis are secular. If they made ba'al teshuvah and became religious, the missionaries wouldn't stand a chance and no law would be needed.
As for refuting Christianity, it is easy to do. I just don't like to undermine Christianity. It is rude.
I would expect receprocity.