It was ridiculous
P3P was useless because it works in exactly the same way as the Evil Bit (RFC 3514).
What happened was:
* Site owner finds their site gets blocked by IE6 in its default configuration
* Site owner adds an arbitrary P3P header which stops the site being blocked (usually copy-pasted from some other site), to make the "problem" go away
There's no penalty for publishing "wrong" policy in P3P. So it's just an arbitrary technical hurdle: it shows that the website owner knows how to add a HTTP header, and penalises website owners who either are non-technical, or else fully understand and reject this nonsense.
I expect Microsoft realised that IE6 was just blocking people from perfectly valid websites, and so it was another reason for users to switch to Firefox.