Exactly. Stripe can't support every bit of functionality out of the box, because then the API would get really convoluted. Just think about what sort of discounts are commonly use: absolute discounts, percentage discounts, coupon codes with expiration dates, conditional coupon codes, bulk discounts, discounts that only apply for the first month. And so on. When dealing with billing logic there is a tremendous amount of accidental complexity.

Stripe offers a low-level API and a higher level API and framework can be built on top of that. So what we really need are some high-quality django/rails modules that deal with all the mundane bookkeeping. Then you can easily fork the module to make adjustments where needed.