I don't have a subscription to Richgel's site, so I can't read his take, but when I look at the USBC page on this I can't find anything that says individual bowlers have to complete this training, only those adults in the volunteer program.
As others have said, just put in language barring junior bowlers from your adult leagues, problem solved. I will bring it up at my fall league's start and recommend they pass it into the bylaws.
It's not unreasonable to make sure the volunteers/coaches complete the training. Individual league bowlers ... um, no. I'm not watching videos so I can bowl.
It's unfortunate. I got my start as a junior in an adult league because I had no other options in our small town. But if it comes down to Safesport training for all, or banning the juniors, ban 'em.
Jess