Yeah but those companies, Coca Cola etc, can only pay as long as Twitter have the user-base. And if the adverts cause users to go visit the site less, then their revenue stream from advertising would shrink.
Would they put averts in the Developer-API responses too? One one hand it's free to use, on the other hand it's being paid for by advertising. What company wants to be a slave to advertising companies? That sounds like a difficult situation to be in.