I consider it as quite plausible that the reason why Intel included ME into all chips is that it is much cheaper to add those unnecessary gates to any chip than to create two different versions of it. The much more interesting question is why ME cannot be disabled. It is clear (see http://www.intel.com/content/dam/doc/product-brief/mobile-co...) why Intel has a reason why ME should not be possible to disable on some chips. I can imagine that Intel fears that if it can be disabled on some chips, hackers will find a backdoor to also disable it on those chips where it shouldn't be possible.
Only "under some conditions" should not be possible, that is, once you as a user turn on the anti-theft protection. Theoretically, turn-on-once, afterwards-no-turn-off technology can be implemented.