> Can’t the detection be optional on the part of the users like do not track option in browsers?
Yes, and the IndieUI API already takes this into account. As currently specified, the user would be prompted on a per-domain basis. Web site authors would have to include a string explaining why they need this information. Users could determine if the reason was justification enough to share their settings, and if not, they would just click “Don’t share” or press the Escape key.
My main argument for the API is that this information can already generally be detected through heuristics. This just standardizes the API and adds a security mechanism so that users can 1) know when a site is trying to user their settings, and 2) deny that access if desired.