But WHY does cannabis cause heedlessness? Other than "it gets you high". I know alcohol limits our ability to make rational choices by shutting down portions of the brain. What does cannabis do?
This reminds me of a story a Thai monk told me. I asked him why he was not allowed to make physical contact with women (his order is very strict) and he told me that in Shakyamuni Buddha's time, one of his followers, a monk, trailed along behind a man and a woman as they entered a temple. The monk gave a squeeze to the woman's rear end or something like that, and this caused quite an uproar, and brought a lot of accusations down upon the sangha and so forth, tarnishing their image.
So, the Buddha took the expedient approach and said, "okay, from, now on, monks are not allowed to touch women (and there are more vinaya rules along these lines). Perhaps one might say this was a bit harsh, or that he'd gone too far, but from his perspective (If i may), what was that monk's reason for signing up? At some point, these 'all or nothing' situations arise. I can imagine him saying to that monk, "look, if you want perfect realization, great, but don't just show up and start goofing off!"
Of course, certainly not everyone -buddhist monk or not- is going to start groping females every time they get near one! But the Buddha laid down the rule for ordained monks. he had to come up with a solution and that was it.
Nobody is required to become a monk, and generally speaking, nobody is required to take precepts (there probably are
some teachers who do require it from their students at some point) but it's not like pork is forbidden or you have to keep your head covered or eat fish on Friday. It's not like "sin" in religion. Buddhist precepts are practical guidelines that apply beneficially to a vast majority of people.
Likewise, not everyone who smokes pot, either medicinally or recreationally,is going to zone out.
I have known a lot
of very devoted, mindful and compassionate buddhists who also used intoxicants.
I knew guys who were dealers and gave half of their money as dharma offerings.
Samsara is one crazy town!!!
But let's be honest...pot plays with your head and oatmeal doesn't.
In some traditions, if you take precept vows like this, and you take them for life (as opposed to, for example, a short period of observance), then you really are supposed to commit to that. It's sort of like getting married. There is nothing wrong for a single person to go through a string of relationships, but if you get married and you make a promise to that person to be 'faithful' then, you know, now you are playing by a new set of rules, and it is important to keep that promise or else things are very likely to get really, seriously messed up. And you don't really want that.
Reading your posts, it sounds to me like there is a conflict, like you want to be a good buddhist but you also want to have the experience that cannabis offers, and the precepts are sort of getting in the way of that. If that's the case, then you probably shouldn't take that precept, just as a guy who likes to date a lot of women probably shouldn't rush into marriage.
There is really no problem, but there is also no point in trying to redefine the meaning of "intoxicant".