well first off, an IP does not mean you can find anyone. In fact, chances are low.
wrong
If someone uses a public computer, or even a computer on a shared home network, like if they have several roommates all sharing the same router, you can't track down one person.
wrong again. many routers track MAC addresses and upstream ISPs log *everything* and they do turn it over quickly with subponea. also, if forensics investigators get their hands on any of the machines invovled they can quickly say whether any of the machines performed the illegal activity as most people are clueless about how to proper cover their tracks.
alright let's walk through a simple scenarios.
1) Someone goes into a local coffee shop, uses the free internet access to do something illegal, and walks away. Investigators track the activity back to the coffee shop. Then what? Tell me how that gets a name or address, because even IF that coffee shop can turn over a MAC address, there's no way to google it.
2) Someone goes into a local library or computer lab that does not require individual logins and does something illegal. Investigators track back to the location. Then what?
3) someone lives in an apartment with 4 other people, all sharing the same modem and router, which is in the apartment. Investigators track back to the apartment. Three of the people are running the same OS and have the same browser. The router seems to have disappeared. Then what?
4) Someone drives around a neighborhood until they find an non-secure home wireless network, and use it for illegal activities. Then what?
I mean, yall appear to be ignoring the fact that it would take law enforcement, NOT an angry lawyer or random person, to demand information from ISPs in the first place, let alone how to track people who are on shared networks, or have enough push to secure a warrant to enter a residence and take computers. So no, someone threatening you on the internet is NOT enough to accomplish that.
If it were that easy, stolen laptops would be returned a lot more often then they are currently. Here's a group talking about tracking:
The Ethical Hacker Network - Track someone using thr MAC/Physical Address?
Why is it you think that google came forward not to long ago with information that they'd been hacked, and no one has been caught yet?