Their free tier has a 100GB/month bandwidth cap — surely the most trafficked websites in the world would very quickly hit that limit, no?

Your point stands nonetheless — they could certainly be losing money on certain large users of their free tier. Serving relatively static content has gotten extremely cheap across the board, though, and like the parent said, they might be benefiting from the pricing by more indirect means.

They probably get away with a lot since these sites are static. Static sites are a wonderful thing indeed.