It’s a matter of supply and demand.

Most of the cloud customers are using it to serve data more than receiving data. Meanwhile, the cloud providers are connected to the Internet via mostly symmetrical links. This means that they are underutilized in the upload direction, and loaded in the download direction. So they charge more in the direction where there is demand.

Download traffic is not the only one that is costly. Traffic between their datacenters from different regions may be billed as well.