Okay, let's try a less "silly" thing. Is altering a picture in an article or a book usually considered a part of normal editing?
Editing happens in all books, but that doesn't mean that all editing is honest or ethical.
Secondly, the point you now agree with, that it happens in all books doesn't mean it's ethical or honest I agree with.
But it doesn't mean that because it's done it's for unethical or dishonest reasons either.
99% of the time I would say it's so so simple as to why it's done. A promotional picture of a mercedes wants to show it is a unique thing. Any reference to any other car in the background would thus be edited out.
Add to that public perception changing over time then if public perception changes to red cars are now out of vogue then the next promotional picture of that car will change from red to some other more in vogue colour.