Complexity without ease of use costs. S3 kind of forces you to use those features. If all you wanted was just a public/private access, you're forced to read into the complexity of S3's complex permission system.
So, to be fair, yes it costs to not use features you do not need in this case.
You literally pay by the hour to use S3, and you also pay the hours you spend trying to understand the permissions and modify settings, so it literally costs to not use the features you do not need in this case. I'm trying to make an argument, I'm not saying you pay much, but I answered your question, and you do pay, either by time, or money.
So, to be fair, yes it costs to not use features you do not need in this case.
You literally pay by the hour to use S3, and you also pay the hours you spend trying to understand the permissions and modify settings, so it literally costs to not use the features you do not need in this case. I'm trying to make an argument, I'm not saying you pay much, but I answered your question, and you do pay, either by time, or money.