Bilge pump wiring...

[hippie]

As most of you may know I put a new engine and numerous other parts in the boat. I am installing a second bilge pump as a backup but ran into something I never saw nor heard of. I bought a brand new Rule float switch to install with a bilge pump I had lying around. The directions on this Rule switch say to install the wiring for the switch on the neg/ground wire. Every set up I saw in my life has the switch spliced into the pos/hot wire. Can their diagram be correct? Is this something new? Please help me out.
Thank you, Bob