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.
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.