Totally fair that you prefer JLC's approach. There's clearly room in the market for different models.
Part substitution is mainly a question of trust. As far as I understand it, you preorder parts through JLC's website, so JLC could easily automatically sub out your passives if they wanted to. For all that people worry about this a lot, I'm not sure if the incentive to do it is really there for shops like JLC or PCBWay. The components that are easy to substitute are cheap anyway, and these companies can easily just pass on any additional cost to the customer. I'm not personally going to haggle with PCBWay over a few additional pennies for a chip resistor or ceramic capacitor, given that the entire service is mind-blowingly cheap.