They've tried to prevent people from publishing shell games that no one actually buys to play, that exist only for the sake of generating keys that e.g. make the discounts on game bundles look more attractive, yes.

But in general what notafraudster said is true, and generating keys to sell is in fact explicitly condoned:

https://partner.steamgames.com/doc/features/keys

> Steam keys are meant to be a convenient tool for game developers to sell their game on other stores and at retail. Steam keys are free and can be activated by customers on Steam to grant a license to a product.

> Valve provides the same free bandwidth and services to customers activating a Steam key that it provides to customers buying a license on Steam. We ask you to treat Steam customers no worse than customers buying Steam keys outside of Steam. While there is no fee to generate keys on Steam, we ask that partners use the service judiciously.

In practice, as long as you're selling a real product and your keys are for legitimate sales then you'll face no restrictions. Pretty much common sense.