So much of this started with one of the Pentagon's more successful marketing campaigns around 2009.  Most of the NFL, MLB, and NBA (even a few Nascar) patriotic tributes were promotions paid for by the DoD.   There were made to look like the team and their fans offering a heart-felt "thank you for your service" of course.

Google Paid Patriotism for the Cliffs Notes version or read the whole report here https://static.politico.com/98/a4/d61b3cae45f0a7b79256cf1da1e0/flake-report.45am.pdf

The crazy side note is that NFL players weren't even on the field for the national anthem before this started.