All the Q&A sites want Google traffic. Google has scraped them all, and thus inside Google's index is a "massive database of questions". Excerpts from this massive database are used by Google to improve its products, and excerpts are often displayed as part of their cached pages or results-page snippets.

Is Google in "a lot of trouble" for their behavior? Should they be?

If they made it seem like the page was written by google, then yes, they would be.

Sure. But whatever Mahalo's done, they haven't made it seem like they wrote the questions themselves. The visual design attributes the questions to the 'fluther' Twitter account (and link to the exact origin Tweet).

Are you angry they're making questions by your users seem like they were written by Fluther itself? That seems a legitimate concern, but your own Twitter stream similarly blurs question authorship.

Are you angry that they synthesized a 'fluther' account on Mahalo and thus made it look a little like you're endorsing Mahalo with your participation? That's what I would be most angry about, if my question-tweets were showing up, under my unique username, at a site I'd never opted into. But one remedy for that concern might then be to remove attribution -- making the questions from 'an unnamed Twitter user'.

But that doesn't apply here, as Mahalo is linking to the Fluther twitter account.