What were they thinking? If a competitor had wanted to sabotage the positive news of DRM-free tracks, they could not have done better than this. The only plausible theory floating around is that this would allow Apple to partially detect casual exchange of music files, since determined redistributors could easily tamper with the embedded information. But it is hard to see how whatever benefits Apple would derive from this would be worth the bad publicity.
Alternatively, this could be an EMI imposition, not impossible given the habitual cluelessness of the majors. But if so Apple should have resisted, if nothing else to preserve their good name.
Why should we worry if we are not set on copyright infringement? There are all sorts of innocent ways for tracks to get to someone, for example when someone (family member, friend) gives them a used computer. An insecure tagging method is open to all sorts of mischief. And the natural expectation is that I should be able to give some DRM-free tracks as I can give a CD. Sure, that might be not with current draconian interpretations of copyright allow, but it is a natural expectation for non-technical people.
Personally I don't need to worry, I'm still buying CDs.