They're not the only company that closely guards their parallel programming requirements
just imagine the flood of posts if they released parallel programming info.

it's likely someone, somewhere will attempt to use it on the wrong IC. The result could range from failure to function to complete and total destruction of the target IC.
There's a problem here, but it's being ignored, not only by the chip makers, but by the user commuity as well
the problem is not 'ignord' but who can change the fact of life that getting experience cost money. I remember a very costly experience I had: "plugging a Nohau adapter into a DIL socket the wrong way makes it pop"

However, if they want to promote ISP as a valid programming scheme, they should publish precise specifications for that, too
they do:
NXP (most): use standard serial and our software.
NXP (the extremely small derivatives): use our 'bridge' and our software.
Atmel: use our cable and our software.
SILabs: use our dongle and our software.

publishing all the timing and voltage level details. will just increase the quagmire of the kitchen table programmers. The sad story is that (I will not name manes) reading through most "kitchen table programmer" threads it is obvious that was such published, the builder would ignore them.

The obvious soultion, in my opinion, is to delete all information re the built in ISP/ICP/IAP/... except use this e.g. for NXP: connect to the srial port and run FlashMagic on the PC THE END

Erik