You're assuming its only the transit cost (as does my original cost); it could also be to discourage use of S3 for mass public serving due to architectural constraints.
My understanding is that S3 is near indestructible from a serving perspective, but it may be computationally expensive for AWS to scale up/scale down for bursty/peaking outbound traffic serving needs.
Another possibility is that Amazon doesn't charge for inbound, but charges a premium for outbound to balance their traffic ratios so they can peer with providers vs having to buy their transit. The free inbound traffic to S3 offsets their outbound traffic not only from AWS, but their consumer-facing web properties.
