From what I've read, some people take heroin in order to feel good; some people take heroin in order to feel nothing ... and some people take heroin because, for them, not taking heroin is too disgustingly horrible to consider as an approach to life.

Well let's assume for the sake of argument that's all true. Then I will rephrase my question as follows:

When people take heroin, they do so in order to feel X. e.g. to obtain pleasure, or to ease their withdrawal symptoms from the last time they shot up, to feel nothing, or whatever.

They value (immediate) X and they are obtaining X in a direct way. How are they simplifying any utility function?