If karma is objective, how come it's cause for "good" rebirth? EDIT: I'll try to be clearer in what I want to know.

The philosophy of good/bad, like/dislike is really irrelevant. 

Question: If karma is neutral, then how come "good" karma leads to "good" rebirth. These aren't my words, I've heard it being said: "good karma leads to good rebirth". What "good" means in the context of Buddhism is beside the point and why Buddhists even use the term "good rebirth" is another question. Weather or not being rich is ultimately a "good" thing, "good" karma still leads, rather conveniently, to "good" rebirth, which I take as meaning rich and healthy since that is the only interpretation I've heard.

But what I want to understand is why doing good can't lead to rebirth as, for example, a worm? Why does "good rebirth" mean to be born generally fortunate (by unenlightened standards) when karma is neutral?

I hope that clarifies.
Okay, I will take a shot. As I understand it...

Karma (Pali: kamma) cannot judge actions because 'kamma' refers to the intentional actions of persons. The result or fruit (Pali: phala) of some intentional act (kamma) is related to that act in a lawful (although complex) way.

We say that healthy habits tend to lead to having a healthy body because of a causal relationship between the action and result. If one exercises, eats healthy food, and keeps her body clean, this tends to promote bodily health. The laws of physics and biology are not judging someone's bodily habits; it is simply the case that, the laws being what they are, 'healthy choices' such as exercise  tend to lead to healthy results (cardiovascular health), and 'unhealthy choices' such as smoking cigarettes, tend to lead to unhealthy results or disease (cancer).

In the same way, according to Buddhism, the nature of the universe is such that certain kinds of bodily, verbal, and mental actions are causally related to certain kinds of results. If, e.g., one initiates good actions, i.e., one intentionally gives, is kind to others, and seeks to learn from the wise, this will tend to produce good results including fortunate future birth. As in the above analogy, the laws governing kamma are not judging you; it is simply the case that, the laws being what they are, 'good/skillful actions' such as kindness tend to lead to good/desirable results (being born beautiful), and 'bad/unskillful choices' such as intentionally killing beings, tend to lead to bad/undesirable results (short life as a human or birth in animal or hell realm).

The relationships between intentional action and its results were well-known, we hear, by the Buddha, who taught them to others. See, e.g., the Maha Kammavibhanga Sutta