People buy into IPOs partially because they know a lively secondary market exists, where they can offload the shares later. Index funds are part of that secondary market.

Just to be clear, I don't think investors in IPOs are thieves. I'm just saying that you can legitimately say that the secondary market financies companies just as much as the primary market does. Perhaps a better example might be farmers selling to food factories selling to retailers selling to me. I never hand money directly to the farmers, but you can still say with a straight face that my purchase of bacon funds the pig farm.