Apparently this was actually a pretty significant case, as it was publicised at the time and led to the creation of laws setting the minimum age for marriage at 16. Although, wikipedia claims he was 24 rather than 22. I feel like this suggests this wasnt really the norm at the time the way the textbook suggests.
https://en.wikipedia.org/wiki/Marriage_of_Charlie_Johns_and_Eunice_Winstead
There are only five sentences of text on that page, with the last one explaining that this sort of marriage was not common at all. Where did you get the idea that the textbook is suggesting that this was the norm?
The second paragraph to the right of the photo talks about how our perception of these things changes with time, and while it seems shocking to us now it would once have been taken for granted. It was a big news story at the time and was not taken for granted.
Edit: I guess my wording was a bit off. I meant to say that it was not within the cultural norms of the time. As worded, it sounds like I'm discussing its frequency rather than its level of acceptance - that's my bad.
Given that the law was passed two weeks after they were married I think it has already been drafted and the whole subject was probably one of much debate. Also note three neighbouring states passed laws at the same time.
It seems likely that someone said "find a story about under-age marriage for our front page", and these two hapless yokels (or should I say Johns the hapless yokel) were the only ones stupid enough to have their photos taken.
In summary, I think society was working up to passing a law like this, and these two had a bride that was younger than most and got married at the right time.
That said, the author has definitely tried to imply that this sort of marriage was commonplace in the 30s, when it was probably at most "unusual".
Maybe an example that gets the point across would be European royalty. When Mary Tudor was six, she was promised to Charles V of the Holy Roman Empire. He was 22, and they were cousins.
While that might not be typical of marriages in England at the time, there are certainly similar cases among the nobility until relatively recently. Enough to make the point about how cultural standards change.
While it is suggesting it was common at the time, it doesn't outright state they're talking about that time. At earlier points in history it certainly was acceptable, but we probably don't have pictures of it to go in textbooks. This reeks of them having a general point to make and having a picture that almost fits that point. I've made more tenuous connections for college papers before.
Also, while it's not as drastic, I was doing some looking into family history recently and I found some ancestors who got married around that time. The marriage certificate listed the wife as 17 and the husband as 21... but the math didn't add up when I found their birth certificates and on the marriage certificate she was aged up from 15 and he was aged down from 22. It was in a small farming community and at that point in time and place schooling was largely abandoned during harvest and as soon as kids were old enough to help out on the farm full time they would just stop with school. And for women, helping out on the farm meant taking care of the house and raising kids generally. Time at school was a waste for them so they just got right to the adult stuff immediately.