Endchan is a platform. But what a platform is?
A platform - such as Endchan, or Facebook, Youtube, Twitter, Reddit, 4chan, *chan... - is a bus. A mod is the bus driver, a dev is the mechanic, users are the passengers.
If one of the passengers stole the wallet of another passenger who would be liable for the act?
The thief, the road the bus runs on, the other passengers, the bus driver, the mechanic, or the bus itself?

A platform is not a publisher. It's not like a news site, a NGO's website, a personal blog. A platform is a service users can interact with and populate it with content, the publisher is the user.
In legal context a platform is an "interactive computer service" and its liability is regulated by "Section 230" (47 U.S. Code § 230 - https://www.law.cornell.edu/uscode/text/47/230).
The freedom of speech is taken very seriously in the United States. If a user generated content was deleted on a platform, the user could claim that his freedom of speech was violated. Section 230 was put in place to grant immunity to platforms for removal content they deem harmful. But to be able to work, grow and be developed they are also not liable for the content users generate.
As Josh put it in an article, the lawmakers provided a sword and a shield for platforms. How I see it they gave them a shield so they can use the sword.

Please note: Section 230 doesn't mandate "interactive computer services" to remove any content. The law only encourages it and protects it.
This was in 1996, and this law made it possible for platforms to thrive. Since then other laws were passed, and future changes can be expected. We will return to them in a later post.