I'll play devil's advocate to this one... you're saying "interactions" are places where ads aren't supposed to be? Isn't any interaction between a business and individuals on Twitter really marketing?
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.