These are IP cameras, that is, a camera which runs a website that can stream whatever the camera is pointing to.
I recently bought one of these to act as a baby monitor. It needs an OS in order to run the web software and Linux is already available, so why not use it? I'm betting the configuration used chops out almost everything apart from the absolute essentials.
I have a Foscam FI8910W. The first thing to note is that initial configuration including a mandatory root password change /must/ be done using an Ethernet cable. At that point you can choose to use the wifi link or disable it; the same with uPnP. Wifi is paired with my wifi router and, as far as I can tell, only by connecting to the router with a MAC-listed client can anyone access the camera.
There is a Dynamic-DNS facility to access the camera over the Wild West Interwebs but that requires configuration which I have disabled as not required.