I believe most nations protect the family of the president or prime minister. It is reasonable to expect that a successful kidnapping or assassination of a close relative would have big impact on the leader's ability to perform his or her job, thus it would seem that it is in the country's interest to prevent this from happening. Plus there is of course the state's responsibility to protect its citizens. Due to his fathers occupation Netanyahu is at more risk than other Israeli citizens, reasonably a higher risk should warrant more precautions.
One student killed, three wounded at Arizona campus during fight (a)
from the article: Israel funded bodyguard to protect Netanyahu's son in New York