Why would European tourists be allowed to take part in the purge?

You behave based on the country you're in (lex terrae, or Law of the Land). Therefore, of course anyone can kill anyone, as long as they are on America soil.

LOL you're really questioning the ethics of a system where the government sanctions a night of murder? Why would they do that? Because capitalism, baby. If the story made any sense, the visas would be very expensive. And obviously, those tourists would be vetted to receive visas in other ways, as well, just like visas are in the real world. Agents of hostile nations would not get visas. Short of direct murder, but such as things are, already, in the real world, things like corporate-driven trade agreements already fist average citizens in various nations. As we speak, the President of the US and the likely next are in favor of, and actively pushing a trade agreement that sells the national sovereignty to multi national corporations: the infamous TPP.

Why can't you be a non-conformist like everyone else?

First, why you complain about if they are European? So, Micronesian are ok?

In the movie they explained that was Murder Tourism in reference of developed countries tourism to undeveloped countries for child prostitution/organ transplants/adoption/murder (that's the premise of Hostel right?)

In this alternate USA things are so depraved at the point that you can even go to Murder Tourism.

If North Korea got together? (And i think you mean North Korea and Ganga Style Korea) travel to "America" (I think you mean United States of America) with one million armed soldiers/disguised tourist with full gear and ammo and bla bla style) (thinking about Red Dawn original and remake / Invasion USA) and the USA customs keeps like "La La Land", then something is really wrong with "our" country.

The whole premise of The Purge is the rock bottom of society morals and politics, it happens to be in USA, because is filmed in USA, but as Carlos says "In Juarez is Purge Day every Day"

This is the opossite at Minority Report.

What if North Korea got together and decided to invade the United States during purge night? Would that be allowed too?

Foreign terrorists have been running around in our country for decades. What's the difference?