When is copyright infringement ok?

It certainly isn’t new , but advertising that plagiarizes other work seems to be a growing trend.
boing boing pointed out an Australian spot for Ricoh Printers that lifted dialogue from MIT researcher, Scott Aaronson.
Here’s the quote from his lecture:
“But if quantum mechanics isn’t physics in the usual sense — if it’s not about matter, or energy, or waves, or particles — then what is it about? From my perspective, it’s about information and probabilities and observables, and how they relate to each other.”
And here’s the commercial:

Now consider this poster I saw on the MTA.

Both are funny. Both use someone else’s words. The Family Guy ad can take the moral high ground since parody falls under fair use. But I think it’s an easy laugh. It might be harder to write an original line, but it’s always worth the effort.