You're right. Unless the funds invested returns anything against it, this wouldn't be investment but donation. I am more interested in Github to find a way out of Kickstarter to create a whole new revenue stream for them.
I'm not sure I would consider the Kickstarter crowdfunding to be "investing". I think "investor" has a certain level of formality attached to it. Investors usually get some stake or something. Most of the Kickstarters I've seen give t-shirts, stickers, access to early betas, pre-orders, etc. Giving to a Kickstarter project seems barely more than a donation. I just think of it as an incentivised donation. If GitHub was to introduce some such feature, it would probably need to have similar incentives as well (bug fixed, feature added, etc) although straight donation (or tip jar or what every you want to call it) would be possible too.