Spigot does this for another reason: thanks to someone making the arguably stupid choice of using GPLv3 for a project linking to (propiertary) Minecraft source

Spigot does this for another reason: thanks to someone making the arguably stupid choice of using GPLv3 for a project linking to (propiertary) Minecraft source code, CraftBukkit has been DMCA'd by one of the lead developers over Mojang taking over the project when the lead dev team quit and is now essentially illegal to redistribute.