Does my computer get involved when person A sends something illicit to person B? As a normal user on the internet, no. That's between them, and the law only deals with them. With certain decentralised anonymous systems, the answer is different. Now there are legal liability issues, at least.
The biggest reason I've never run a Tor exit node, either back when I was running a hosting company or now that I have unmetered gigabit (with 10 gig available) is that I don't want the feds to come knocking about CP and the like activity on my IP.