"XfinityWifi" and "PublicWifi" are being added to my list of networks without my knowledge or consent (OS/X El Capitan ... MacBook Pro)

From time to time, when I go to *System Preferences...Network...WiFi...Advanced," I find that two network-names have been added to the list without my knowledge or consent:


XfinityWiFi
PublicWifi


How do I know this? &nbsp; Heh ... "Because I happen to live in a hole." &nbsp; That is to say, "a little corner of Northwest Georgia (USA)" where there is exactly one wireless network: "Mine!" &nbsp; I work from my home, so my computer rarely leaves it. 

I have turned off "Remember networks that this computer has joined," and in any case I can positively say that this computer has never encountered, let alone "joined," either of these networks.

So ... what's going on here?!
Wireless networks are synced across iCloud, so if any other device such as an iPhone connects to these networks and is also signed in to the same iCloud account as your Mac, the networks will appear in this list.