Precisely because of it. On Linux, /bin/bash, //bin/bash and /bin//bash are the exact same file, the same inode. They look somewhat off to people, but they're entirely harmless, so cleaning that up is an aesthetic choice, not something important.

On S3 they're different. Using the wrong paths causes weird issues, like not finding things you expect you find, or storing multiple versions of the same data out of sync.

Normalizing // to / means making S3 behave more like people expect.