Reading the replies it seems you're getting a mixture of "I agree / understand" and "It's a business decision you feel wrong".

Humans have weird triggers for emotions. Seeing something that benefits a group that you are not a part of but fairly equivalent to is a typical trigger, even if it does make sense to a degree. When I first saw this, as a non-YC-member, I felt pretty similar. I'm sure it's simply a business relationship in which either digital ocean is making money due to the advertising or YC is subsidizing it (though the latter is unlikely).

So it makes perfect sense from a business standpoint but I think your feelings are equally valid. I would love to see digital ocean partner with other incubators or provider offers to any start-ups in general.