Well, of course, you're right, but if we look that google will try to move content sites to the top and have paid advertising around then this is exactly how content site visitors would behave in normal situation.

And again quality score consists of 23984 elements so this could be just one of them.

On one site they spend 30 seconds, but on another site they spend 3 minutes - why? because they using some advanced comparison tool that makes them to hang out there.

About 30 minute opening - well, its quite tricky one, you need to page trough pages and have some activity. I bet they can come out with some nice ideas for this;)