Whether we have or have not printed an invoice/purchase order is useful for us to see so that we do not have duplicates or be missing documents that can make it confusing or mean we miss something. Clicking on the email button and having it then show up as printed as well, despite it not being printed is not useful.
I suspect the problem is that I guess the "print" mechanism has no idea of the "device" being used for the print. So when you email you actually print it to an email "device"; this "device" sets the emailed flag, but the "print" process then sets the printed flag as it has printed the item.