Why can I waitpid() but can't EVFILT_PROC under pledge("proc")

[Luke Small]

What if I want to prevent a process from forking while I want to create new
EVFILT_PROC events? Say, to accept the pid of a sibling fork from a pipe
and load it into a kqueue. Is there a reason why waitpid() isn't beholden
to this, or is there a reason that EVFILT_PROC is?