Also, it's just a good look towards the community. Remember, ideally not all of ATProto's infrastructure is run by BlueSky themselves, but by a diverse set of folks who want to be able to control their own data. These are more likely to be individuals or small groups, not capitalized startups. While the protocol itself is designed to be able to do this, "what happens when BlueSky is huge and running your own infra is too expensive" is a current question that some folks are reasonable skeptical about: it's no good being a federated protocol if nobody can afford to run a node. By doing stuff like this, the BlueSky team is signaling that they're aware of and responsive to these concerns, so don't look at it as trying to save money on bandwidth: look at it as a savvy way to generate some good marketing.
That's my take anyway.
EDIT: scrolling down, you can see this kind of sentiment here: https://news.ycombinator.com/item?id=41637307
Also, this was a fun thing I built mostly in my free time :)