I think there are a number of reasons for this. First and foremost I would say that mainstream opioid addiction through pain pills, especially illicitly obtained pain pills, is a relatively new trend. Heroin as been the most demonised drug for decades now and as a result just has a much greater stigma.
I also think IV use is a factor. I would suggest a much higher % of regular heroin users IV compared to other opiate addicts/users. As this is the highest risk ROA it carries a higher stigma and this influences peoples perception of heroin.
Pharmaceutical companies have done a damn good job of promoting popping a pill as a quick solution, I believe subconsciously society on a whole views popping a pill as a lot more normal and less "junkie" than sniffing a line or putting a spike in your vein.
Finally, the uncertainty and possible contamination of dope compared to quality controlled pharmaceuticals influences the perception of danger a lot. Rightly so, almost all heroin OD deaths could be avoided if the user knew the purity.
There is definately less stigma attached to the pill scene than the H scene, is it justified? In my opinion it can go either way, I mean from reading Bluelight it seems a lot of modern heroin addicts started out as pill addicts, as a result it is probably reasonable to conclude your average heroin addict is further down the depths of addiction than your average pill popper. However, it has to be judged on a case by case basis, and someone is certainly no more or less moral because their opiate of choice is heroin. One interesting thing to point out however is that a heroin habit seems to be much cheaper to maintain in the states than an illegitimate pill habit, it may well be the case in future (or now for all I know? I am in Australia) that a higher percentage of petty crimes will be attributed to pill users than heroin users due to the higher cost.