OS X 10.6 aka Snow Leopard. They introduced Grand Central Dispatch, which allows software built using Cocoa and Objective-C to handle small lumps of code off to the daemon which decides where and when to run it. Unsurprisingly, it can also enforce restrictions on said thread and kill them if it steps outside of them.

This is how they can halt (or kill) the main thread of a process while keeping a background thread running.