Why do I get spam on Twitter?

Factual Questions
1

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?

2

Here are the Twitter rules. Scroll down for the section on Spam and Abuse:

http://support.twitter.com/articles/18311-the-twitter-rules

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.

3

I thought tweets that don’t have a legitimate origin are ads?

4

Yeah, on re-reading I think the OP must be referring to promoted tweets:

https://support.twitter.com/articles/142101-promoted-tweets

There’s a Dismiss button.

5

So it’s spam but they came up with another name for it.

6

More like it’s an ad, masquerading as a tweet.