Well, good for them. My high school biology teacher was inspired to start what is now a highly successful health products company by doing something similar.
FWIW, in those days ALMOST ALL teachers worked during the summer or after hours to supplement their nine months per year income. And, for the most part, we had better and more qualified teachers. The duds found other professions where they could work less and pay more.
Sadly, so did some of the superior ones like my high school biology teacher. But even he is serving society in a far better way now. And I'll bet his compensation reflects it.
My grandfather worked at summer camps, and then gave golf lessons when he was older.
Doctors often work several jobs.....they never quit...and they don’t retire after 30 yrs....