In some way these guys are "investing" in these YC companies' future, hoping that when they grow they will stick with DO. YC has good track record so it's relatively a good bet, whereas if they did it with any rando startup, they'll probably lose tons of money (statistically 99% of the startups die). Remember this is a company not a charity.
If your company employs less than three people then $X amount of service provided per month.