Cloud providers are commodity, and egress pricing is partially cost following because they have to pay peering to their interconnect points for WAN. Internal networks are not charged within the account because the economics of the VPC overlay are optimized for that use, but inter account and VPC and other boundaries carry cost - especially interconnection between accounts because the way VPC treats virtualization requires a relatively expensive routing. Inter AZ and inter region pricing also exists for the same cost following reasons. They also help shape incentives because it allows them to optimize placement of compute within the same AZ to physical buildings or rings.

The case that is largely nonsense is the egress pricing on direct connects since beyond the circuit costs, which the customer pay, there’s no costs for aws not already on the customer regardless. It also makes DC friction weird in that you are incentivized to NOT move storage before compute.