[Everything in this post is purely my own personal opinion, and may not reflect the opinions of everyone working at Mojang Specifications!]
Large parts of the culture these days exists in a world where copies are free. Copying a physical book costs money, but copying a digital movie is free. In fact, simply moving a movie from one hard drive to another actually copies the movie first, then deletes the original. Copying games is also free. No resources are lost, nobody loses any money, and more people are having fun.
To people who want to get paid for their digital works, myself included, that is a bit of a problem. All of society and economics is based on an old outdated model where giving something to someone would rid the original owner of their copy, so everyone who wanted a copy had to buy one from someone else who would lose theirs, and the only source of new copies was you. There might be actual development costs involved in making these copies. For example, for every wheel in the market, someone had to make that wheel. With digital copies, you only need to make the wheel once.
I won't bother analyzing why people copy games and other digital media, as that's really a moot point. We've got an amazingly effective way of distributing culture that is extremely beneficial for humanity, but it clashes with our current economical models. Piracy will win in the long run. It has to. The alternative is too scary.
If someone pirates Minecraft instead of buying it, it means I've lost some “potential” revenue. Not actual revenue, as I can never go into debt by people pirating the game too much, but I might've made even more if that person had bought the game instead. But what if that person likes that game, talks about it to his or her friends, and then I manage to convince three of them to buy the game? I'd make three actual sales instead of blocking out the potentially missed sale of the original person which never cost me any money in the first case.
Instead of just relying on guilt tripping pirates into buying, or wasting time and money trying to stop them, I can offer online-only services that actually add to the game experience. Online level saving, centralized skins, friends lists and secure name verification for multiplayer. None of these features can be accessed by people with pirated versions of the game, and hopefully they can be features that turn pirates from thieves into potential customers.
Please don't interpret this text as me being fine with people pirating Minecraft. I'd MUCH rather have people pay for it so I can reinvest in hiring people and developing more cool games in the future. It's also quite possible that if I get into a business deal with a larger company, there might be a larger push towards fighting piracy mostly because they'd require it, and I understand why they'd want that.
But why fight the biggest revolution in information flow since the printing press when you could easily work with it by adding services that actually add some value beyond the free act of making a digital copy?
In other news, I'm voting for either Piratpartiet or Miljöpartiet in the Swedish elections on Sunday.