From section 3.2.1.
Annotations are likely to get a lot of people with a lot of money really annoyed.
This means that generally, not too many people who have the funds to support the global annotation system have great interest in it. Moreover, third-party web annotation may become the issue of many copyright lawsuits. It is unclear who wins these trials, but the inevitability of these is surely a deterrent. Here' marker on their web site. This could be similar to the robots.txt file that is already used to limit the activities of web spiders.