One of the reasons this distinction is important is that because of the way human brains are designed, thinking about your preferences can cause them to change. Furthermore, this phenomenon is more likely to occur with high level moral preferences, then with low level amoral preferences.

If that's a definition of morality, then morality is a subset of psychology, which probably isn't what you wanted.

That's not a definition of morality but an explanation of one reason why the "murder pill" distinction is important.