Why must Tor Browser be on Safest?
Safest turns scripting off, closing the most reliable route to break anonymity. Awazon still works fully.
Short answer. Safest turns scripting off across every site, which closes the single most reliable route anyone has to break anonymity from inside the browser. Awazon is built to work at Safest.
What the level changes
The security level does not change the Tor network, which is the same at every setting. It changes what the browser does with a page once it arrives. At Safest, scripting is off everywhere, and scripting in a hostile page is how nearly every serious deanonymisation has happened.
Why it costs you nothing here
Some ordinary websites break at Safest because they lean on scripting. Awazon does not. Login, browsing, checkout and messaging all work with scripting off. Set the dial to Safest and leave it there.