For one thing, I have an iPhone, and I have a car with Microsoft Sync. Every time I plug the iPhone into the car's USB port, it pops up "this accessory is not made to work with iPhone". And then it works anyway (except for the part where something is a little overzealous about reindexing the music on the phone, and I'm not yet sure which side that one's on).
For another, the DRM point may be more important than the author makes it out to be: there are plenty of systems where using the "wrong" cable will still work, but with significantly degraded access (I still remember the first time I had to buy an adapter box to use a DVD player with my TV). I'm not sure that "sorry, you can't do that" is any better or worse than "you can do that, it just won't be useful in any way".
It may be called "degraded usability" for some...
The white ones always had Firewire 400 too (http://www.apple.com/macbook/specs.html). There was a brief period when the 15 inch Macbook (later renamed Pro) had no Firewire.
The Air, predictably, doesn't have Firewire. It barely has USB...