It's my impression that Europe has been a little bit more relaxed than "only idiots are traveling". It's only now that the numbers are rising again that they're again putting increased measures in place.

Yes, it's correct, it was possible to travel between EU countries during the whole summer without that much restrictions (some places required a quarantine of 14 days, but others didn't). The number of identified cases only started to rise again in central Europe around mid August.

Why do you think there is a magical virus barrier at the border?

It does not matter if you travel to the next city or to the country nearby. Yes, there are clusters but at least in Europe we do not have vast empty areas between populated places. It is pretty much a continuum.

Travel creates new unidentified clusters, the tourists also tend to leave the area rather quickly => very hard to find and trace who originally caused the cluster, and who were in contact.

Tourists also tend to have much different riskier behaviour compared to the average native inhabitant of a city (eg nightclub hopping many days in a row)

If you're travelling a short distance over a mostly open land border then sure. If you're flying somewhere then it's a different story.

There isn't a magical virus barrier at the border. But there's a jurisdictional barrier there. Contact tracing across the border is much more painful than doing it locally. Our civilization-level immune system is bureaucratic in nature, so you can say that national border is a barrier for its antibodies.