If A means not B, then selecting for A is only the same thing as selecting against B IF A doesn’t also mean other things, besides not B.
In the dating example, a (straight) woman might employ positive selection to choose men who are particularly decent people. This would also have the effect of weeding out thieves and rapists (assuming that the woman in question can assess a man’s decency with sufficient accuracy), but the quality of “being a decent person” doesn’t only mean one isn’t a thief and a rapist; it’s more wide-ranging than that.
I don’t understand how using friendly behavior to reinforce people visiting one’s desk precludes that behavior being genuine. You seem to be dismissing the possibility that the person in question feels real affection, and is smiling because they are in fact happy that their desk is being visited. Just because they are using their (real) positive response to coworkers visiting their desk as positive reinforcement doesn’t mean that their behavior is “fake” in any way.
Just like a woman who feels a surge of affection towards her husband when he puts away the laundry, and kisses or praises him.
Yes, it’s positive reinforcement, but it’s also a genuine response.