1) They want to be an ISP
2) They feel bad for America and want us to enjoy faster internet.
They're doing it because they have plans for products which necessitate nationwide high speed internet. I don't know what those plans are... maybe making YouTube the everything-video-delivery-system, maybe massively collaborative work tools as they expand Google Docs et al... maybe something we haven't even heard about... but they've got something coming that they require Fiber for.
This is a business plan.