I’ve read the twitter rules and I can’t tell if they allow spam or not, it seems like they don’t. But at least once per day I get a tweet that is not from someone I follow or a retweet. This is not allowed right?
Here are the Twitter rules. Scroll down for the section on Spam and Abuse:
You can block a user and report them for spam.
As for the question of why you get spam in the first place: it’s what spammers do.
I thought tweets that don’t have a legitimate origin are ads?
Yeah, on re-reading I think the OP must be referring to promoted tweets:
There’s a Dismiss button.
So it’s spam but they came up with another name for it.
More like it’s an ad, masquerading as a tweet.