Well, the problem with the first scenario is that if the discount is percentage based (say, 10% off), then it's going to require code to calculate. It's no longer just an SKU - it's an SKU which does a calculation, which is presumably not part of the platform already. And so we have a new "discount" SKU - but which line items should the SKU apply to? How do we know when to stop using that SKU when the discount runs out? And anyway, why should an SKU have a discount % field on it? Why do we get different behaviours based on some magic SKU field? It's a breakdown in cohesion... and so the spaghetti begins...

You can also argue that we can just "get someone" to apply discounts manually, but isn't that the job of the billing system? Manual labour doesn't scale too well, and a lot of mistakes get made - asking a customer to return an accidentally applied discount is a very, very difficult conversation, and I've worked with companies who simply had to absorb years of incorrect discounts because doing so was the only way to keep the customer.