While some of these cameras are meant to be public (such as weather cams or traffic monitors), many are accessible simply because they lack proper password protection.
If your interest is genuine (e.g., watching traffic, weather, or tourist spots), there are legal and ethical sources of free public video: inurl viewerframe mode motion my location free
"Location-Based Motion Tracker"
Known as the search engine for internet-connected devices. You can search by map or by device type (e.g., "webcam"). While some of these cameras are meant to