I see this as a parallel to the question of `pathlib.PurePath.resolve()`, about which `pathlib` is (rightly!) very opinionated. Just as `foo/../bar` shouldn't resolve to `bar`, `foo/` shouldn't be truncated to `foo`. And if `PurePath` doesn't do this, `Path` shouldn't either, because the difference between a `Path` and a `PurePath` is the availability of filesystem operations, not the identities of the objects involved.
On another level, I think that this is a simple decision: `PosixPath` claims right there in the name to implement POSIX behavior, and POSIX specifies that `foo` and `foo/` refer (in some cases) to different directory entries. Therefore, `foo` and `foo/` can't be the same path. Moreover, `PosixPath` implements several methods that have the same name as syscalls that POSIX specifies to depend on whether their path arguments end in trailing slashes. (Even `stat` [http://pubs.opengroup.org/onlinepubs/9699919799/functions/stat.html], which explicitly follows symbolic links regardless of the presence of a trailing slash, fails with ENOTDIR if given "path/to/existing/file/".) It feels pathological for `pathlib.PosixPath` to be so almost-compliant.