And yet other pictures of famous people in these textbooks were/are in color. There is a visual discrepancy in presentation and it misleads the viewer (children in this case) in a way they may not even realize for years or decades after. Whether this particular discrepancy is purposeful or not it is problematic.

In addition to color being too expensive for textbooks, it was also too expensive for newspapers. And colour film was more expensive than black and white film. Since photos taken by photo journalists at the time were meant to be printed in newspapers in B&W, most photographers shot with B&W film even while the technology for colour photography existed.

the sentiment of black-and-white photography being the “true” way of documentation

Well... B&W does have better resolution, both back then and now. Notice how many photos from NASA probes are in B&W? It's because to get color you either have to take three photos with filters on them and combine them together, which is what NASA does. Or have clusters of three different sensors in an array to pick up the different wavelengths, which is what most consumer cameras do. But that effectively cuts the resolution into a third of what it could be if you had sensors that simply detected light without caring about the wavelength.

Of course the way most cameras are constructed you don't get any benefit from B&W in terms of resolution since the way the sensors are arrayed is optimized for colour. But NASA's cameras allow for higher resolution B&W images (when they already know the colour of the thing they're looking at and they want to see detail) and the filters are there when they need to figure out what colour something is.