Interesting, in my country spam is very much illegal and I would hazard a guess that it is also illegal in the US, similar to how littering, putting up posters on peoples buildings/cars/walls, graffiti (a form of spam), and so on is also illegal. If I received the amount of spam I get in email as phone calls I would go as far as calling it harassment, and of course robot phone calls are also illegal. Unsolicited email spam is also again the law.
And if spam is against the service agreement on twitter then that could be a computer crime. If the advertisement is fraudulent (as is most spam), it is fraud. Countries also have laws about advertisement, which most spam are unlikely to honor.
So I would make the claim that there is plenty of principled reasons for banning spam, all backed up by laws of the countries that the users and the operators live in.