The Move/Copy Sheet dialog always defaults to "Move", even if you have literally just performed a Copy action. It should remember the last action you did and default to that. This is especially notable when you are working within a single document and moving/copying sheets, as a sheet that is "moved" within one document can lead to data loss if you don't realize that you didn't make a copy, and edit the "new" sheet.
There is request in bug 68582 for split Move/Copy item in menu. With resolving of bug 68582 this bug is also solved. Closing as wontfix.
*** This bug has been marked as a duplicate of bug 68582 ***
I don't see why is this WONTFIXED or DUPLICATED by a different proposal, which itself got WONTFIXed eventually.