Why teach C using scanf without talking about command line arguments?
I think that the rationale is so you don't have to introduce arrays and array notation on day 1. Typically, arrays enter into the picture at roughly the same time as loops, which would take place a few weeks or months later, depending on the pacing of your course.

That said, if you want to start with command line programs, it's not a huge problem to do so.  At the very, very beginning of programming, there are always a few magical incantations that students must type, but won't fully explore until later.  args[0] could easily be such an incantation. 

As a side benefit to this approach, your students may have an easier time when they come to arrays, since they've already used them a bit.