I don't think you're interpreting the terms correctly. Using the logo to promote the platform is the total purpose of it. Now I understand some companies like Nintendo do not allow this, but that is because their brand name also carries a seal of approval in quality.

However, I think if I were to make Steemit pins and t-shirts with the logo and sell them, then I would need to seek their permission.

Using the logo to promote your own content on Steemit is fair use of the logo in my opinion.