As mentioned already, there is a positive connotation associated with doing something for somebody. However, this is because doing something for somebody often implies that they wanted it to happen to them in the first place, whereas doing something to somebody implies that you wanted it to happen to them, regardless of whether or not they enjoyed what you did.

Even in the earlier examples used, there is a difference between

The wicked witch made a poisoned pie for the children.

and

The wicked witch gave the poisoned pie to the children.

It is the latter case in which the children are directly affected in any negative way. The former case implies that the witch did something laborious, even, while the children remained unaffected. If the sentence were left alone without a follow-up, we would never know what happened to the pie, and would probably assume that nothing bad happened at all, and it was the witch that wasted her time (due to the underlying notion that she did something for someone else at her own expense). It wasn't until an action happened to the children (i.e. they took the pie from the witch) that the children were negatively affected.

Again, doing something to somebody is not always perceived in a positive light, because it represents what the doer wants to happen to the receiver. In the case of saying hello to somebody, we assume that this is a good thing, because we are unharmed by greetings and generally enjoy receiving them. But that greeting could have just as easily been a malicious one.

As another example, when you do something positive at your own expense, it generally is followed by a 'for'. This is represented in the simple fact that you do not say 'I did something bad for him', but you would say 'I did something bad to him'. On the other hand, you would typically say 'I did something good for him', whereas you wouldn't say 'I did something good to him.' Take the below example:

Alice cut the cake for Brad's wedding.

When you read that, you assume that there is a delicious cake that Alice is cutting a slice off for Brad and his friends to eat. Thus, Brad and company are able to eat the delicious cake. But what if this was not the intention? What if Brad had ordered a beautiful wedding cake for his future wife that his jealous ex-lover Alice was cutting into smithereens? It seems unlikely, based on the use of 'for' in the above sentence. Also consider the cognitive dissonance that results from this sentence:

Alice cut the cake for Brad's wedding. Then she threw it into the garbage.