..
By "smooth", do you mean the selection borders? If so, that's natural. Photoshop is not really designed to be a pixel editor and thus will antialias whenever possible, since jagged, sharp edges are unwanted most of the time.
Gradent Black-transparent would prolly be a better description. in 1920x1080 pixels i dont think there are much rough edges.
Its ether a bug, or some verry stupid idea from the programmers side. 
I think you might be misunderstanding.
Photoshop smooths selections. If it didn't, the result without any shadow of a doubt would be rough looking, regardless of your resolution. This is called aliasing.
Here's a circle I made by using the elliptic selection tool and filling it in:

Is the problem present in this image? If no, then...
...here's the same circle made with the elliptic selection tool, but with one of its settings changed:

Do your selections look something like this? If so, the problem is actually your fault, not Adobe's. This is caused by a setting known as "feathering" and is entirely off by default. You changed it at some point. To disable it, just change to the selection tool, look at the topmost palette, and set Feathering to 0.