The reason you can access most online video content via youtube-dl is that historically speaking, YouTube and most other Flash-based video players actually had no DRM at all. Flash's DRM support was crappy, unreliable and expensive to license and few places used it. I think I've literally come across one site that used their proper DRM, at which point I discovered it didn't actually work on my PC. (A handful more use RTMPE, which is hilariously broken but enough to block youtube-dl out the box.) DRM's becoming a lot easier with HTML5, and a bunch of sites that didn't previously have robust DRM are now planning to start using it.