Interesting question that you are asking yourself. For me I would recommend anyone to experience the pilgrim's mass regardless of any beliefs or none.
Do you think that you will go next time?
Ron "Butch" Briery wrote the first guide to the California Mission Walk. He lives locally to me, and has walked a few Caminos too.
https://missionwalk.org/the-hikers-guidebook-second-edition/
Cicerone has a guide regarding California Mission Trail. To be honest I have not read this seeing as I am not planning this trail, but would be curious to see what it has to say. Good luck whatever ever path you take.
Is it possible the people who buy these are tourists who really do not know the genuine reason for deserving a Compostela?
The tourist thinks "I have paid 30 euros therefore I can get meaningful souvenir along with a fridge magnet".
This site is run by Ivar at in Santiago de Compostela.
This site participates in the Amazon Affiliate program, designed to provide a means for Ivar to earn fees by linking to Amazon