That's a valid point, but as an internet user the domain is the only attribute visible to me that can determine a site's identity. So, I am pretty happy that the W3C insisted on setting the privacy rule based on plainly visible attributes, not on hidden predetermined or site informed data.

You call the current policy ambiguous, but the proposal wouldn't even ensure that all browsers block or allow the same sets of domains.

Anyway, I am aware that people use multiple domains on the most diverse ways. But it's important to have the technical behavior simple and predictable, and if Google doesn't like the consequences of the way they decided to organize their domains, well, it's their problem for them to fix, not a reason to push complicated unreliable standards on everybody.