Most Fediverse software supports image uploads, so there's no reason to use Imgur for image hosting. Hell, even on my small single-user server (atomicpoet.org), image hosting is easy peasy. Not only is Imgur not needed, they're an annoyance for those of us who are used to seeing images natively on the Fediverse.
I know habits are hard to break, but just remember: this isn't Reddit 🙂
That and to prevent megaleeching by scraping. It's easier and cheaper to purchase curated lists of IPs than it is to bootstrap a tool to ban them based upon bandwidth.
Reduce attack surface/abuse. There are plenty of services that maintains lists of “bad IPs” or “VPN IPs”. You can subscribe to them and get the list and block it. Most bots, spammers, attackers etc always run behind some type of VPN or Tor depending on what you’re doing and how much you’re trying to hide your trail.
Most services don’t go out of their way to block VPN IPs because it’s just a support headache/nightmare. Some services like Netflix are legally obligated to block well know VPN IPs