It strikes me more as just a protection against the case of a massive company being born in the fellowship.
If they just put the cash up for people to work on interesting things and it helps grow the YC network and feed some interesting companies into the YC Core program, then it's probably worth it without any equity compensation.
But that model wouldn't look so great if the next Google comes out of the program but doesn't do the YC Core program, resulting in a massive outcome that YC doesn't get to participate in.
This model supports the former and protects against the latter, in a way that seems really founder-friendly to me. Founders could take the money and build a small business, go to YC Core, give up altogether, or leave the YC ecosystem and build a massive company – and regardless which path they take, this capital won't cost them anything unless they build something massive.