At startup, Impala validates that it has access to the filesystem (via JniFrontend::checkFileSystem()). It does this by doing a getTotalNumVisibleFiles() on the root path. The root path is defined by the fs.defaultFS property. On S3 and other object stores, that is always the root of the bucket.
For security reasons, a user may want to restrict an Impala instance to a subdirectory of the s3 bucket (or other object store). This check at the root on startup makes that impossible.
Impala should provide a way to customize where the check takes place. The check is primarily about making sure that Impala has appropriate permissions and is configured correctly. Checking a subdirectory of the s3 bucket is just as effective for this purpose.