Of course it is. Heck, just high school cheerleading is a huge business.

That's not the same meaning of commercial used in the linked clip. High school sports are not a profitable activity for the organization running the team (the school), and building nicer facilities doesn't bring them significant monetary benefit.

Unfortunately, one reason high school sports get funding is that, just like college sports, it tends to be instrumental in prompting other sources of funding (eg. donations from alumni). Or at least that's the perception.

Like at prep schools? Or some huge football program in Texas? This definitely isn't the case at the vast majority of schools in the US.

I think you may be underestimating the prevalence of the phenomenon. Try some searches like "high school sports alumni" or "high school sports donations".