Hi,
two possible answers:
1) Some employers want to hire people who seem well educated, and well educated native speakers should usually be able to use a less "regionalized" variety at work, use a more formal register, and expressing their thoughts precisely, if the employer prefers you to do so.
I'm not saying those who say "hey, waddup y'all?" are uneducated. I'm saying educated people are usually able to change register if required to do so. Picking up the phone at home: "Yo, what do you want, Fred? - At work: "Hello Mr. Picky, may I help you?"
2) It is discriminated against because... why not? There's always a good excuse to discriminate against something. It's not different from sexual and racial discrimination, because those are all still present.
NOTE: sometimes it might be "standard" English the one that is discriminated against. For example if you have a job interview with Snoop Dogg as a producer and you want to get popular as a rapper. In that case talking white might sound too educated and would probably be unacceptable. You might even get shot by one of them gangsta, so you better watch out. LOL