I don't think it's so much that starting a new process in a signal handler is an intended use case, but rather than the post-fork environment is very similar to the signal handler environment, and so calls that work in one will work in the other.

(In both cases, you can't count on currently acquired locks ever being released. In a signal handler, this is because it could be acquired on the current thread, and post-fork, this is because all other threads have been killed from the perspective of the child process. As such, no lock-based code can safely be called.)