Unfortunately, it's only advisory. The government should set up a 5000 NOK (at least) "traveling fine" per person, and obligatory quarantine ("home" confinement in state-provided guarded quarters) upon entry. THAT would discourage people from traveling. So those who MUST travel can plan ahead and know what they're up to, the rest will give up. AFAIK, insurance companies announced that they'll refuse travel-insurance…
So rich people will shrug it off and travel anyway? What about the poor people who MUST travel? What if a family member is dying in another country? Just make a case-by-case judgement. Holidays are clearly stupid, but there are plenty of much more valid reasons to travel.
Hell, Aussies aren't even allowed to leave the country without approval from the government at the moment.