There’s nothing new or odd about restricting travel to hostile countries. I’m not sure why this is such a surprise to you. We may not be engaged in continuous military activities against certain countries, like North Korea, but we may still be considering them belligerents and thus don’t want any aid going to them through private citizens.
And note that travel isn’t prohibited, it’s restricted. That means we can’t just fly to Cuba on vacation (and spend tourist dollars there), but we can apply for permits to travel for family or humanitarian reasons. This would even apply to North Korea if they didn’t automatically forbid all US/US Allies from crossing their borders.
The issue with Moore is that he failed to receive approval before going. Not that he was likely to get it, but that doesn’t make his actions any less illegal.