In this case "positive" is not something "good". It's not there to soften the word "discrimination". It's similar to "blocklist" and "allowlist": you could argue that a blocklist is just an allowlist for each item that's not on the blocklist. Positive discrimination for a trait is negative discrimination for any other trait, but sometimes you'd like to highlight that you select for something, and not that you select against a myriad of other thing.