I don't get it

I frequently peruse the job listings on Craigslist. There are many jobs that list "Blueprint reading a plus" or "must be able to read blueprints". These are not jobs for nursing or phone answering (where the skill is not necessary, obviously), but instead, they are jobs for working on a construction site or as an estimator or a project manager. Who can't read a blueprint and still has any idea what they are making? A blueprint is a picture, to scale, with some ridiculously obvious and logical conventions, and perhaps some notes, of the thing you are trying to make.

Did you see that phrase? the thing you are trying to make. How would you even think that you could do that job if you couldn't read a drawing? And if you can read (really, that's a necessary job for pretty much every job, and how did you get to a CraigsList posting without that very basic skill?) and figure out how to make the thing, and use a measuring device, how could you not be able to read a blueprint?

I understand that sometimes you have crappy drawings to work with, or they don't all match up, and there is some skill in analyzing a drawing, but I don't really see how this is even something you would put in a job listing or on your resume. I don't write "can do math through pre-calculus" or "mastered high school algebra" on my resume, nor do I add "successfully uses a scale rule on a daily basis" or "familiar with Cartesian plane geometry". That is assumed. Kind of like how you figure that I can read and write to the level that is demonstrated with my resume and cover letter.

For those of you out there in the construction field, or who write such ads, what's up with even mentioning blueprint reading?