Earlier quoted context omitted.
There's a lot of unstated meaning wrapped up in "arbitrarily" there. You can obviously treat customers differently based on whether they pay or not, and you can obviously have terms that say what you can and can't post on the platform (and apply moderation accordingly). Otherwise you couldn't maintain any kind of forum or communication platform. Saying you can't post links to other services may be distasteful, but sh…
As I said: "treat them differently for things they do that are neither an explicit breach of contract, nor illegal behaviour" In other words: these rules need to be explicitly stated in whatever contract consumer and provider agreed on. These rules also need to be universally enforced. Rules that are (intentionally or not) written in such a way that they lend themselves for arbitrary application (that is, not equally…
If you want to limit the ways your service (or product) can be used, you can do so by explicitly arranging these limitations through a contract. Things that are already (otherwise) illegal do not have to be included in this arrangement (hopefully for obvious reasons).
Whatever is not explicitly forbidden by either law or contract, are not valid grounds for treating consumers differently.
Sadly, and mostly for historical and cultural reasons (and law enforcers not paying attention for a long time), it is often assumed that these rules do not apply to big service providers (and software producers alike). However, I have never found any legal basis for why such companies should be treated any different than any other one.