It's extortion. It really is no different from a mafioso coming up to your place of business to demand dues owed for 'protection'.

I wish I had the foresight to created idea/feature patents like 'Allowing users to reply in a forum-like setting in order to generate even more conversation'. That way, I could sue Y-Combinator for 'profiting wildly from my idea.' Perhaps the good folks over at Y-Combinator can take a page from your book and give me a percentage stake in every single start-up they help to seed.