Imagine if Bell was touchy about letting "undesirables" use their service.
It’s not. It’s about individual private publishers, of which there are potentially infinite. That is the beauty of the internet.
EMail is one such example: there is no meaningful way to filter spam, so the peering relationships between mail providers break down unless they each implement some sybil-proofing on their own side. The end product is that everyone is forced to use a provider like Google, and play a corporate-political game to stay in good favor with their peers. This is because there is no spam-resistance on Email application level
We've seen a similar situation this week with a controversial forum called KiwiFarm taken down by DDoS. They need to maintain a political relationship with their DDoS-protection provider, CloudFlare, who also controls a massive chunk of the internet infrastructure. This is all so that there can be a central entity that discriminates on the traffic, as there's no spam resistance features on IP level.