The drug relapse rate is
between 40-60%, but the media paints a rosy picture about the stories where drug addicts are able to stop using permanently and are able to change their lives (with help from Dr. Phil, Intervention, etc, of course), and therefore, people believe that a majority of drug addicts are very capable of treating their addiction (especially if they have a "support system," which might not be possible for some due to socioeconomic circumstances) and the majority of drug addicts end up conquering their addiction.
I think the other end of the spectrum is probably more likely, which is that drug addiction (especially heroin) is a downward spiral, and the people that get sucked into it are probably naturally disposed to wallow in whatever it is they are using the drug for, IE: a high, an escape, a thrill, etc. Moreover, the resulting behaviour of drug addicts can be quite detestable, which might be why some responses seem to be heartless. Druggies can be assholes, because for some of them, their #1 priority is getting a high. Everything else, including relationships, might just be a means to that end.