The real problem is that the standard requires a browser vendor to behave in this way. Microsoft is implementing the standard, but arguably shouldn't since it appears to be outdated and flawed to begin with.
About 10 years ago I worked on a site that put together a P3P policy, it was a nightmare. Like many other sites, we needed multiple P3P policies. Your ecommerce site probably has different things it stores compared to your marketing pages, for example, and you certainly don't want your marketing or blog pages saying that you are storing name/address/credit-card and scaring the crap out of people wondering how you are even getting that info off their system.
You can still find the decade-old tools over at p3ptoolbox.org that help you build a P3P policy, God forbid you try to build it by reading the spec. It also seems ironic that p3ptoolbox.org doesn't have a P3P policy and hasn't been updated since 2005.