Imported assigments carry some of their properties with them, including whether they are visible. It is not unreasonable though to want to make some of the properties (eg the release date) different to what it was in the class copied from. However, if the assignment copied was visible in the original class, it instantly becomes visible in the new class, until the instructor has managed to changed the settings. This could result in students having access to assignments they shouldn't be able to see yet. Having an assignment temporarily hidden instead of visible is less likely to cause a problem, as the assignment probably wasn't there before it was imported, so it not being visible to the students doesn't change anything.