Arguably, but there are social bounds to what's considered acceptable, as well as bounds enforced by Twitter to prevent abuse of the medium. Any phone conversation B2C is similar, but in general, we don't like businesses calling us on a false pretext just to get our attention. It's like those annoying snail mail envelopes designed to look like a federal government notice so you'll open them.

Even if all business->individual interactions are marketing, that doesn't mean that all marketing is spam. Marketing and customer engagement is fine. Spam isn't.

The distinction for me isn't between marketing/non-marketing but between humans and robots. If a human representing some business favorites my tweet because they feel that it's worth favoriting, that's great. In fact, it makes me feel good that some human at an organization is reading and caring about what I have to say. Wonderful, I write things so people can read them.

Robots just going out and marking things as favorites just to get attention and followers? That's the kind of spam actively undermines the value of the example above.

That's a non-sequitur: marketing and advertising are not the same thing.

Can you provide a definition of either that excludes the other?

Easy. Advertising is a subset of marketing. Marketing also includes product design, customer service, research, etc.