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.
This is another reason why you should always hire contractors who have employees and not those who use subcontractors. If the company you hire uses only subs to paint, they have no control over the training those subs receive. However, even if you choose a contractor with employees, this does not guarantee that those employees receive training. The sad fact is that training just isn’t in the budget for most contractors.
Not only do I agree with what you are saying; but, I will not use Angie's List for referrals again. Their "A" rated painter did some of the exact things they are now warning against; however, they are still rated "A". I sent in a review and it took eight weeks to post it. They allowed the contractor to lie about what happened as a response. I had proof and photos. Angie's List is a scam; instead ask neighbors and friends for referrals!
These are some fantastic questions to ask and I particularly like the one that is concerned about safety for the painters. After all, if you have a large home and you’re hiring them to paint the exterior of that home then they’ll likely be on scaffolding. Because of that, you need to make sure that the contractor not only provides appropriate safety training, but has the correct liability insurance as well.