I think the Jet containers are just inhailers. Though it's only been 36 years since Myron first made Jet, 2241 (Fallout 2) to 2277. The way I always rationalized it is that the Brotherhood Outcasts took to selling it the locals, because they don't seem especially self-sustaining and they don't really care about what happens to people that aren't in their little group.
Though I suppose by that same logic you could have Brock Flowers and Xander roots growing in/around Oasis. But the Nature of Harold's transformation is borderline magic in it's effects. You could throw almost anything plant related at Oasis and it would kind of make sense. Spore Creatures? They'd probably be Worshiped like the Ghouls in Broken Steel. Desert Plants? Harold brought the seeds over with him on his clothes and his Radiation/FEV cocktail allows them to survive out of climate.
If nothing else it would give a reason to go back to Oasis once you're done there. Which is a problem a lot of the places in Fallout 3 have. There's no reason to go back to anywhere. Girdershade, Bigtown, Republic of Dave, Temple of the Union/Lincholin Memorial. Once you've done the quest they offer they may as well return to their home planet never to be seen again. It's jarring compaired with New Vegas, where you find yourself stopping at NoVac, Goodsprings and Primm over and over because They're just on the way if nothing else. They've also got shops. And usually more than 1 thing to do. Except Primm. Primm is kind of useless, but it still has multiple quest tie ins.