What this basically means is that you can't kill a thread. You can ask a thread nicely to kill itself, but you can't otherwise control its execution once it's started running. The Unix process model, on the other hand, has a mechanism to send a signal to a running process and have it terminate after it tidies itself up. It encourages monolithic programs. Componetized programs are the Real Deal.
Threaded programs are hard to debug. .
Any thread can stomp on any other thread's memory, you have the problem of hanging locks, etc. With processes, you have no stomping, and locks can be auto-released by the system when the process exits. Unix was the original component-based operating system, up until the day that the X window system came around and ruined it. Okay, so threads suck. This is the part the user interacts with.