Perhaps a more apt term would be freeware.

The BUSL is not OSI approved and places restrictions around usage, in an effort to sell some product or service off of a project and prohibit competition. It is not free as in beer either, if you hit such usage.

The source is available (hence source available being another terminology) but even Hashicorp has dropped "open source" from nearly every relevant page in understanding that their products no longer have an open source core (see instead "Vault Community" or "Terraform Community").

The general consensus is that no, the BUSL is not OSS as OSI has not blessed it as such: https://opensource.org/licenses/

Thanks.

> Am I misunderstanding something?

Yes, source being available is not enough to be called open source.

It's worse in some ways because if you read the source and then work on another open source project in the same field you could end up in legal trouble.

It's true that Hashicorp's source is still available, but you might not have permission to use it or modify it any more. Even if you can read it legally. Under their new license, Hashicorp can essentially revoke any commercial user's license to run Hashi software, and there is nothing the business can do except pay. They can do it at any time, if the business has any divisions that to anything close to competing with any Hashi product.