Windows have a visible border when unmaximized, which is necessary to tell their boundaries apart when overlapping, this is hidden in their maximized versions.
The "snapped" windows should also have this border as there can be two apps running side by side, and its necessary to be able to tell their boundaries apart.
Example screenshot of Nautilus and Gedit running side by side, snapped:
Windows have a visible border when unmaximized, which is necessary to tell their boundaries apart when overlapping, this is hidden in their maximized versions.
The "snapped" windows should also have this border as there can be two apps running side by side, and its necessary to be able to tell their boundaries apart.
Example screenshot of Nautilus and Gedit running side by side, snapped: