Politicians from all parties want to associate their brand with "tech." It is associated broadly with pro-growth, innovation and entrepreneurship.
YC is not just any old office space. Holding the event here allows the DNC, and the president, to claim support from this community.
It is a smart political move for the fundraisers. It is also a valuable contribution from YC leadership to the DNC.
Unless there is equal time given to equally important elected officials from other parties, YC can now be branded as a partisan group. You are no longer above the fray.