Answer 1

New versions of the init man page taken from the sysvinit package are saying that usage of SIGPWR is discouraged, since /dev/initctl control channel is the preferred way of communication.

Answer 2

The name of the game is portability. Not everyone’s init handles that kind of signalling gracefully. What’s more, some admins might want to do things differently even if they have that kind of init running.

So, to be compatible, upsmon just invokes a shell command. If you want to use init’s SIGPWR stuff, just put the right "kill" line in a shell script and make your upsmon call it. Everyone wins.