My students and I have discussed this phenomenon and related techniques over the years. Painters have for a long time used optical tools to inform their work...before photography they were known to use a "camera obscura". Heck, even the "invention" of one-point perspective in the Renaissance was considered cheating by some. You guys know this history already, but it is fun to review it!
Anyway, you are right! Some painters use photos to create their work. Many will use multiple images and sometimes composite, edit, distort, etc. to get the look they want. Others eschew the use of any photos as dishonest
Copyright and plagiarism issues is a separate topic, but I encourage my students to use their OWN photos if they go this route. If not, they need to "borrow" the work of others with caution: there are right and wrong ways to go about this (think of the AP Photo and the Obama "Hope" poster as an example).
One of my favorites, Edward Hopper, is a interesting case in point. He use a wide variety of tricks to create a final work of art. Some of his subjects are actual locations-- painted from observations. Even then, his notebooks are full of sketches which often become watercolors and then, sometimes, a finished oil painting. He tried using a camera for a time but was unhappy with the perspective and uncompromising reality of film. I suspect the binocular sensation and perception of our eyes was what he was trying to convey.
There is a book called "Hopper's Places" that shows on-site photos of many of his paintings. Things in these photos have changed over the years, but it is fascinating to compare what Hopper SAW and what the camera recorded