Back to the article. You can add water to all latex based paints / thinner to oil based paint. The tinting base has absolutely nothing to do with it. Say you are working outside and throughout the day you have to add a little water to keep the same consistency. If somebody really tried to add 20% to 50% water they no longer would be painting they'd be performing a whitewash or pickle finish.
Consider purchasing supplies personally to save money. Ask the painter for a bid that separates labor and materials. Then explain that you'll purchase the materials and ask for a list of exactly what will be needed to complete the job. Caulking, for example, is an extra supply commonly used to fill any cracks or damaged areas in your walls -- and one that might be overlooked in an incomplete list.
Did you even read the article? It was specifying UNSCRUPULOUS painters! And, by the way, the photo at the top was not identified at all. How would anyone know whether it was done by a homeowner or not? Also did you ever stop to think that if a consumer has the knowlege to spot a dishonest contractor then by default he also has the knowlege to identify an honest one as well? And, pardon me, but just because you've never seen something has absolutely nothing to do with whether it has actually happened to someone else. Why would any honest business person be so defensive about the publishing of such useful information? If any painters/painting contractors object to a consumer having this kind of information maybe they are the dishonest ones!