Are you not supposed to use an Amazon logo for a link to Amazon?

For the logo linked above, almost certainly not. See e.g. https://advertising.amazon.com/resources/ad-policy/brand-usa... :

> The Amazon logo and the Amazon Smile mark are not permitted in third-party advertising by vendors and sellers linking in to Amazon. The “available at Amazon” logo is permitted for use in advertising.

Here's the "available at Amazon" logo: https://m.media-amazon.com/images/G/01/AdProductsWebsite/ima...

The main smile logo variants are also permitted for use in "logo line ups", "in a line up with two or more other retailers and not linking in to Amazon."

Most other uses are either not permitted, or subject to conditions, which sometimes involve obtaining written permission from Amazon.

That's just an example, other Amazon logos for other product areas - e.g. their app store - have similar conditions.

There may be some fair use exceptions to all this if used in a context that doesn't involve selling products.

Not a lawyer, but surely something can be both open source and trademarked? As far as I understand those are unrelated concepts. In this context, the “source” is the SVG markup.

The issue is how you use it. Whether the icon SVG markup is open source isn't very relevant if you're not allowed to use it. If you want to personally mess around with it and not use it in any commercial context, no-one is going to care.

I guess they crawled all icons from github projects and did not check every icon... I would also double check the license with the source before using it.