First of all there's no need to get so ****y! I asked a question without malice. There is most certainly a difference between tipping an employee, who relies on tips for a good portion of their income, and a business owner/entreprenuer. Tipping a hotel bellhop, airport red cap, taxi driver, etc. is not quite the same and I certainly object to your assumption that I would stiff these folks. The private guide has his/her own company, advertises their service, sets their own fee ($800 in this case) and relies on happy customers referring their service to others. And as a "new cruiser" who has never used a self-employed tour guide before, I think I asked a reasonable question.
Tipping can often be confusing and there are no clear cut rules. But in general, restaurant owners, beauty salon owners, hotel owners, or any business owner should never expect a tip for providing good service but you should tip their helpful employees! There are many things that don't make sense, like why would some professions deserve a tip while others don't, like the medical professional or your accountant? I'd appreciate hearing others' viewpoints.