Although the NPC case is probably about some random people on twitter that like to make fun of some targets, the story about Russians seems to have more involved actors.
What got those 1500 banned were tweets like "Get out and vote November 7th!" 1 day after the actual date of Nov 6th and the boosting and retweeting among all the accounts.
I think the real reason they got banned is that under every tweet of every blue-checkmark liberal on Twitter you'd find dozens of replies of accounts called NPC209834029 with the same avatar nodding and saying the same platitudes. But of course, when all the replies to all Trump tweets are from blue-checkmarks shitting on him, Twitter doesn't seem to care. :P I think neither should be banned, but I guess they know better.
Which violates the Twitter policy against intentionally misleading election content. Pretty straight forward.