Okay, just in case you need some backup, the musical division of the Dear Rich staff consulted with Stephen Fishman, an expert on the public domain. According to Steve, it doesn't matter if it takes great skill or musical training to create the musical notation of public domain sheet music; nor does it matter if the end result is digitized. The work may only be protected by copyright if the sheet music contains substantial additional music, is an abridgment, or involves making a new arrangement -- for example, creating a suite for several instruments with harmonizations not previously associated with the public domain work.
How then can a music publisher claim copyright in public domain sheet music? According to Fishman, many claims for copyright in public domain music are improper and based on the fact that music publishers have a strong economic incentive to convince the public that its music is copyrighted (even when it is not). Of course, this information won't prevent you from being sued; it just means you're likely to prevail if there is a lawsuit. In addition, it may sometimes be difficult to separate the public domain version from a popular derivative version -- as in the case of the folk song "Tom Dooley". (By the way, the rules described here may not be the same outside the U.S.)
As for your question about whether the copying is ethical, the Dear Rich staff is unable to comment as they are trained only to wrestle with legal issues. However, you may find some advice here.
Got a question for Dear Rich? Send it to dearrichquestion@gmail dot com, and make sure it has the header: "Question."