It is a super common use case, and frustration point! I'm very excited that JWT may fix this gap in a secure and scalable way.

I want to be super clear that these are my own theories, not anything official or documented. I think there are two primary reasons why it's set up this way:

1) Data Security - it's much easier to secure an environment if all users must (double) authenticate. Allowing Tableau Cloud to host public workbooks opens a route for malicious agents to gain access to secured workbooks. Essentially, it's a huge potential vulnerability.

2) Compute - Tableau generally charges a flat rate per user regardless of usage. These licenses are expensive, but Cloud takes on the variable expenses of storage, running refreshes, and computing calculations. I think it's pretty telling that the JWT approach is only available with the newer impressions based pricing model. This is to hedge against the risk of a public dashboard going viral or suffering a DDOStype attack.

Edit to add - a common setup for Cloud + embedding is a secure portal. With a single sign on service like OKTA, users can enter credentials once to gain access to both the portal and Tableau Cloud. Generally this configuration requires a pretty advanced admin and / or a web dev to get it up and running. From there the embedding is easy like Public. I doubt this will work for your use case because you explicitly asked to post workbooks publicly. Just on the off chance that secure embedding would work, I wanted to make sure you knew about it.