Are you saying the AI will rewrite its goals to make them easier, or will just not be motivated to fill in missing info?

In the first case, why wont it go the whole hog and wirehead? Which is to say, that any AI which is does anything except wireheading will be resistant to that behaviour -- it is something that needs to be solved, and which we can assume has been solved in a sensible AI design.

When we programmed it to "create chocolate bars, here's an incomplete definition D", what we really did was program it to find the easiest thing to create that is compatible with D, and designate them "chocolate bars".

If you programme it with incomplete info, and without any goal to fill in the gaps, then it will have the behaviour you mention...but I'm not seeing the generality. There are many other ways to programme it.

"if the AI is so smart, why would it do stuff we didn't mean?" and "why don't we just make it understand natural language and give it instructions in English?"

An AI that was programmed to attempt to fill in gaps in knowledge it detected, halt if it found conflicts, etc would not behave they way you describe. Consider the objection as actually saying:

"Why has the AI been programmed so as to have selective areas of ignorance and stupidity, which are immune from the learning abilities it displays elsewhere?"

PS This has been discussed before, see

http://lesswrong.com/lw/m5c/debunking_fallacies_in_the_theory_of_ai_motivation/

and

http://lesswrong.com/lw/igf/the_genie_knows_but_doesnt_care/

see particularly

http://lesswrong.com/lw/m5c/debunking_fallacies_in_the_theory_of_ai_motivation/ccpn