YouTube videos are actually downloaded into browser cache! aren't they streaming?

Question

This may sound a very silly question but has been bugging me for a while! you see, I am able to protect my videos from being downloaded by setting up a red5 media server and using rtmp protocol... then why YouTube videos are actually downloadable and I can find them in browser's cache?! does that mean they are using progressive methods? why don't they use streams then? and if it's a stream, then why we can download them?

Answer 1

Progressive download is better than video streaming. Since a huge number of users will be watching a number of videos it becomes very difficult to manage the streaming server. For which they will have to deploy a huge amount of effort, resources [compared to progressive download method].

Answer 2

By using progressive download it allows seeking backwards without having to re-fetch the data. Seeking forward is nearly instantaneous if that portion of the video has been loaded. By storing it locally you can re-play without downloading the video again, lessening the load on the server.