I would think that there are probably several reasons...one may be that they just don't know any better as you suggested. But bear in mind these quilt alongs are generally free. They are written by everyday quilters like you and me, for fun. You're not paying for a pattern so you kind of have to remember they are doing it for the fun of it, they are not professional pattern writers.
I have done a few quilt alongs, tutorials, and given away free patterns for simple quilts on my blog - I am by no means a professional and someone who is more concerned with the frugality of their fabric as well as their time might knit pick at my cutting methods. But it is just something I do for fun, it's a hobby. ;)
Also they may assume that you have enough knowledge to do your own math and figure how many strips of any given fabric you may need to cut 150 2 1/2" by 4 1/2" bricks.