To serve in the knesset you should be made to make a pledge of allegiance to the state. If you are loyal to the state you would not visit enemy states unless on official (sanctioned) government business. If you are not then you are a traitor. It is that simple.
Knesset passes 'Norwegian Law' allowing ministers to resign their seats, remain in cabinet (Haaretz)
from the article: Arab MKs: Bid to bar trips to enemy states is 'racist'