This isn't entirely true. Regardless of whether or not cannabis is indeed a "weed" is irrelevant. It needs a lot of water actually. I mean, I guess it can merely survive on low water levels, but to produce large plants with lots of resin covered buds, proper hydration is a must.
For example, last season where I live in the Northeast (not a desert) it literally did not rain from mid June til mid July with average temps in the 90's. I was out there bucket bombing at least once a week during that time period. One time, as a matter of fact, I got there just in time as one of my best plants was starting to wilt. During that time, my plants were just surviving, not exactly thriving.
Now I know to take the water situation much more seriously. I suggest anyone looking to do a serious outdoor grow does so as well.