[zahlman]

That does explain the use of the term. They are reasoning about the systems in terms of what will happen (its procedure), but still describing systems as compositions of parts (as nodes in an AST, effectively) rather than as lists of instructions. And they teach how to analyze problems in order to design systems that way. And, of course, writing the program is inherently a self-directed imperative; you can't simply learn how to write programs and have programs exist as a result — you have to take that initiative.