The policemen weren't shocked that the gate closed. It was recent news that earth would soon become a mature world, meaning all the while they've been in some sort of tutorial.
Now, once there was a dungeon break and the boss was killed outside the dungeon, it would close. Meaning the reason blue gates that had dungeon breaks didn't close in the past was so that they could extract resources and fight one creature endlessly to grow, but now that earth would soon be considered a mature world, that leverage was gone.
Unfortunately, this also had its drawbacks. A dungeon break was dangerous on any level; it meant either a casualty or devastation of property, which was a loss either way. Guilds, freelancers, some large corporations, and the authorities might not really want gates to be closed down since money was being made from them.
