In my posting on the uses of the present continuous in the Archives I wrote:
"This use of the present perfect continuous ("someone has been eating my chocolates") is based on present evidence of recent past activity. It focuses more on the effects of the activity than on the activity itself."
This does not mean that when there is present evidence of a past activity the present perfect progressive must be used. Other factors can influence the choice of verb tense and aspect. For example, if the action is seen as a single completed event, the present perfect is used:
Someone has taken the last brownie!
Sally has ruined my CD player!
Either the present perfect or the present perfect continuous is correct for the example sentences, but the implications are different. With
You haven't eaten for days, have you?
...the implication is that no food at all has passed the lips of the hearer. This could have been voluntary, but it may very well have been involuntary, due to a lack of food or resources for obtaining food.
You haven't been eating for days, have you?
...the speaker is talking about suspension of the habitual activity of eating. The decision to suspend that activity may very well have been voluntary--a decision made by the hearer.
Here are two possible scenarios to illustrate the differences:
1. (To your cat, who has been missing) Oh, sweetheart, you look starved! You haven't eaten for days, have you? (falling intonation on the tag question) Poor kitty! Here, let me get you something nice to eat.
2. (A doctor to a patient) Mrs. K., you look starved. You haven't been eating for days, have you? (falling intonation on the tag question) I've told you that you don't have to lose any more weight.