> We are talking about a totalitarian regime which does not care in the least what the West thinks
You are wrong. First, totalitarian regimes care _a lot_ about what the West thinks. I know it because I grew up in one. They have to be just scary enough, look crazy enough, and display some signs of "progress" enough (like — can anybody in the West organise Mass Games? Look at our highly efficient society!) Modern Russia is, unfortunately, now falling in the same behaviour pattern.
Now, of course, tourists that are allowed to contact foreigners, especially Westerners, are filtered through layers and layers of state security. But it is impossible to do thorough filtering. You can't stop kitchen talks (they attempted to do it in Soviet Union and East Germany, but failed). Despite you always are in well-staged Truman show, it is not perfect, and real world is always showing through cracks, and most importantly, they can see _you_ through cracks.
Back in USSR, in town of Omsk in Siberia where I grew up, a foreign tourist was a legendary event to be talked about for months (which is why it is of uttermost importance to travel beyond capital cities, where life is less staged and opportunities of random encounters are more numerous) — and if he managed to leave some artefacts, like US-made pen or postage stamps, they were caressed as real treasures, the evidence of world beyond the wall.
And, of course, smugglers and missionaries are also doing their jobs (VHSs with Hollywood movies also played their part in the fall of USSR). But not everybody can be a smuggler or missionary. Everybody can be a tourist and do their part, though.
And about the money — as a tourist, you'll hardly help the evil regime with more than $1000-$2000 (do you know how much a rocket costs? This is a small change). North Korean government is not doing it for your money. They want to look good in your eyes, to pretend they are not that evil (see that other comment in this thread — they want more people to think that way).