Why wouldn't it be a choice?
Oh, it absolutely *is* a choice, I believe.
But, on the other hand, I believe that addicts have poor decision making skills and an inability to properly conceive of and weigh consequences.
For example, an addict who's been clean for a while knows that he can cut loose and have a dose, and still not have to worry about WD's at all. Hell, might as well even have fun and go on a weekend binder, right? The worst an addict would have to face from an extended weekend of abuse is maybe a little watery eyes and a runny nose. No big deal, of course, and a very small price to pay for such a great weekend.
But as (bad) luck would have it, it's been an awful week. Why not just use a few more days to get yourself over the hump--sure, the WD's from a week-long binge will be noticable, but it's not like you'll be hunched over the toilet and flopping around like a fish. Maybe a few rough nights of sleep, but after all that's over, it's back to clean living--a good workout regimine is the only drug I need after I opiate myself from this particularly icky week.
But talk about bad luck! Just when you've resolved to clean up, you have to put in over time all next week. I'm not 19 anymore! No way in hell am I gonna bust my ass for 10 hrs a day when I'm detoxing too much to get a good night's sleep.
The potential WD symptoms keep getting worse and worse (would you actually quit), but alas, the shit you're running into in life keeps getting worse and worse. You realize you're strung out, but now your resolutions no longer involve quitting outright (as you're painfully aware that, by this point, CT is out of the question), but rather to taper yourself clean.
And on those rare stints when you actually DO manage to cut down a bit, it's been so long since you've made any progress that it's occasion for celebration--then *poof*, the tiny bit of progress you've made has been entirely erased: Fuck, you're in even deeper.
IDK if this is how all addicts think, but I'd say it's a pretty good summary for me. The basic principle here is an inability to accurately weigh how hard quitting will be (ie, the "consequence") against how much enjoyment you'll get from getting high (the "benefit").