As a quick response, I would say that it is less "mechanics don't work like fluff tells" and is more like "mechanics don't touch what fluff says."
The fluff says that gangers take it to get a "better high," not a longer one. You are right in that there is no resistance against the effects of most drugs. But this doesn't mean that Enabler isn't doing what the fluff says it does. Getting a "better high" from drugs isn't really covered in the mechanics of the drugs, it is a fluff aspect of using drugs. There isn't a stat on the drugs that says "how good a high" a character gets from taking it, so the fact that Enabler doesn't modify anything in that regard is unsurprising.
Now, I haven't looked at the rules for Enabler, but does it also modify addiction ratings or thresholds? Because based on the wording you mentioned I wouldn't be surprised if it also made the user more susceptible to addiction. But that might not be covered in the rules effects either...