Yeah, I'm confused about it as well. It seems like the candidates are the ones getting most of the value out of the program, so they should be the ones paying, right? And that "founder associate" role they hype up, it sounds like it's more like a "founder's assistant" gig than anything, which still sounds misleading, because I'm sure these people get little to none of the upside of being a founder.
I think that getting startups to pay helps align everyone. Otherwise there might be a conflict of interest between Jumpstart, candidates and said startups. This way, startups can make sure that Jumpstart works for them, fulfilling their needs.
This is important given the value prop, it's definitely a lot more risky to take on juniors in these kinds of environments. If juniors were paying, Jumpstart might be incentivized to help them despite it not being a great fit for the startup.
Companies which focus on vetted skilled individuals have more flexibility if the brand is trusted. They can charge candidates, ask for % fee of 1st year salary etc. while still providing an acceptable risk.