Why not? He's broken laws and enabled others to do the same, he's profited personally from breaking various countries' laws. Are you saying that copyright/intellectual property owners and content creators are owed less protection or right to redress under the law than individuals consuming content? In the UK the police under the POCA powers would have rightly seized all of his assets.
It wouldn't be so bad if MU had abided properly by the DMCA and Safe Harbor frameworks, but they wilfully ignored it and therefore became wilful infringers, something courts look far more dimly upon.
It's the domino effect. If you leave something like mass digital piracy checked, it quickly becomes completely unmanageable as we saw last decade. The scale of Schmitz's profiteering from ignoring the rights of other individuals and companies became hard to ignore, and curbing his company's behaviour became more and more important. His actions were having a measurable, deleterious effect on the market and ecosystem: not just less money being made, but fewer artists coming through the system and ultimately, less music, film and other media being brought into the world.
There was a ring of several particularly bad offenders, but MU along with Rapidshare were by far the worst for a few years. MU was the most obvious target because he made himself the biggest target (physical size quips notwithstanding). He's the modern day classic fraudster, a digital conman.
In my younger, more naive years, when I was a normal Joe Bloggs consumer, originally I was concerned about how the US pursued Schmitz and MU (even though he was obviously no angel). However, having worked in the independent music sector, which was hit hard by last decade's upsurge in piracy, I witnessed, and had to deal with, piracy's effects first-hand. Over time my opinions changed accordingly and I have actual experience behind what I say.
I keep in contact with other people from similarly small indie labels and discuss this stuff periodically. The problems definitely haven't gone away, just lessened slightly. Fortunately now there's better tools in place for people to once again legitimately enjoy music and films, but overall the market is still much, much smaller than it was in 2000, even taking into account things like how people's listening habits have changed and the methods they use to watch TV.
There's a key difference between dumb filehosts and the companies like Megaupload whose primary profits came from distributing copyrighted material. Music, film and games have an intrinsic monetary and cultural value over and above (for example) an ASCII art text file or picture of your dog, though all of those things enjoy the same protection under intellectual property laws. However, the cost to produce a movie or record an album is vastly more, and by reducing its market value to zero, you irreparably devalue it in the eyes of those who would otherwise pay a fair price to enjoy it.
Instead of paying a few quid and enjoying the films and games, customers become simple consumers of content - hoovering up everything they come across with no regard for its value. Once that happens, it becomes a horrible race to the bottom as society effectively declares that all the things they enjoy are not actually worth anything.
Eventually the supply of quality entertainment just dries up. Crowdfunding and a handful of self-released albums only go so far to filling that void when nobody's willing to put down the money to support not just the creation, but the marketing, retail and distribution infrastructure on which everything depends.