It's an important issue, and one that seems impossible to resolve.
At some levels, all human communication can be seen as advertising. Discussing whether equal amounts of matter and antimatter were created when the universe began (if it began) can be considered advertising for one side or the other.
Defining advertising is therefore critical to deciding what advertising is acceptable.
I see a lot of posts that are advertising, in the widely acceptable definition. People advertise ICO's, cryptos, businesses, groups, and political positions constantly on Steemit.
A corporation is far more easily defined. However, telling a corporation from a person on Steemit is pretty much impossible. We can't even tell bots from people.
There are folks on Steemit that frankly state they are professional censors. @bloom's about me blurb states they are a 'paid flagger'. Questions on how to deal with such issues are bound to be difficult to answer in a way that doesn't prevent free speech.
I reckon free speech doesn't include paid speech, and that's where I'd draw the line, myself.