Why haven't they been shut down?
 

Why hasn't Limewire been shut down like Napster?

The Limewire software is not illegal...
Limewire has no servers, it's just everyday people sharing files from one and others computers...
Limewire is not responsible for what the users share...;)

So, that wasn't what Napster was? What was the difference between Napster, then, and Limewire, now?

This may help explain...
http://en.wikipedia.org/wiki/Napster

Okay, so it was the same, but now programs like Limewire are "decentralized" so they are harder to shut down. I guess I get it, but I wonder what they mean by decentralized.

No center. No big central mainframe to seize and shut down, like Napster.

Napster actively participated in copyright infringement.
LimeWire never did so, not even indirectly.