It does actually manage to read index.json, it's com.ubuntu.juju:released:tools.json that causes the problem. Guessing, but I'll say it's to do with colons in the name.

--

2015-06-26 11:55:37 DEBUG juju.environs.simplestreams simplestreams.go:429 read metadata index at "https://s3-ap-southeast-2.amazonaws.com/78ac3a374f2f4c498475863a430c9b1f/tools/streams/v1/index.json?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAJETX5WILMFNUU5BQ%2F20150626%2Fap-southeast-2%2Fs3%2Faws4_request&X-Amz-Date=20150626T195536Z&X-Amz-Expires=604800&X-Amz-Signature=5155b5f06f78363d3c33c87af4332c64419f432208d8a8492a05b1cd59229af8&X-Amz-SignedHeaders=host"

2015-06-26 11:55:37 TRACE juju.environs.simplestreams simplestreams.go:617 candidate matches for products ["com.ubuntu.juju:centos7:amd64" "com.ubuntu.juju:centos7:i386" "com.ubuntu.juju:centos7:armhf" "com.ubuntu.juju:centos7:arm64" "com.ubuntu.juju:centos7:ppc64el" "com.ubuntu.juju:15.10:amd64" "com.ubuntu.juju:15.10:i386" "com.ubuntu.juju:15.10:armhf" "com.ubuntu.juju:15.10:arm64" "com.ubuntu.juju:15.10:ppc64el" "com.ubuntu.juju:12.04:amd64" "com.ubuntu.juju:12.04:i386" "com.ubuntu.juju:12.04:armhf" "com.ubuntu.juju:12.04:arm64" "com.ubuntu.juju:12.04:ppc64el" "com.ubuntu.juju:win2012hvr2:amd64" "com.ubuntu.juju:win2012hvr2:i386" "com.ubuntu.juju:win2012hvr2:armhf" "com.ubuntu.juju:win2012hvr2:arm64" "com.ubuntu.juju:win2012hvr2:ppc64el" "com.ubuntu.juju:win2012hv:amd64" "com.ubuntu.juju:win2012hv:i386" "com.ubuntu.juju:win2012hv:armhf" "com.ubuntu.juju:win2012hv:arm64" "com.ubuntu.juju:win2012hv:ppc64el" "com.ubuntu.juju:win8:amd64" "com.ubuntu.juju:win8:i386" "com.ubuntu.juju:win8:armhf" "com.ubuntu.juju:win8:arm64" "com.ubuntu.juju:win8:ppc64el" "com.ubuntu.juju:win81:amd64" "com.ubuntu.juju:win81:i386" "com.ubuntu.juju:win81:armhf" "com.ubuntu.juju:win81:arm64" "com.ubuntu.juju:win81:ppc64el" "com.ubuntu.juju:13.04:amd64" "com.ubuntu.juju:13.04:i386" "com.ubuntu.juju:13.04:armhf" "com.ubuntu.juju:13.04:arm64" "com.ubuntu.juju:13.04:ppc64el" "com.ubuntu.juju:13.10:amd64" "com.ubuntu.juju:13.10:i386" "com.ubuntu.juju:13.10:armhf" "com.ubuntu.juju:13.10:arm64" "com.ubuntu.juju:13.10:ppc64el" "com.ubuntu.juju:14.04:amd64" "com.ubuntu.juju:14.04:i386" "com.ubuntu.juju:14.04:armhf" "com.ubuntu.juju:14.04:arm64" "com.ubuntu.juju:14.04:ppc64el" "com.ubuntu.juju:14.10:amd64" "com.ubuntu.juju:14.10:i386" "com.ubuntu.juju:14.10:armhf" "com.ubuntu.juju:14.10:arm64" "com.ubuntu.juju:14.10:ppc64el" "com.ubuntu.juju:win2012r2:amd64" "com.ubuntu.juju:win2012r2:i386" "com.ubuntu.juju:win2012r2:armhf" "com.ubuntu.juju:win2012r2:arm64" "com.ubuntu.juju:win2012r2:ppc64el" "com.ubuntu.juju:12.10:amd64" "com.ubuntu.juju:12.10:i386" "com.ubuntu.juju:12.10:armhf" "com.ubuntu.juju:12.10:arm64" "com.ubuntu.juju:12.10:ppc64el" "com.ubuntu.juju:15.04:amd64" "com.ubuntu.juju:15.04:i386" "com.ubuntu.juju:15.04:armhf" "com.ubuntu.juju:15.04:arm64" "com.ubuntu.juju:15.04:ppc64el" "com.ubuntu.juju:win2012:amd64" "com.ubuntu.juju:win2012:i386" "com.ubuntu.juju:win2012:armhf" "com.ubuntu.juju:win2012:arm64" "com.ubuntu.juju:win2012:ppc64el" "com.ubuntu.juju:win7:amd64" "com.ubuntu.juju:win7:i386" "com.ubuntu.juju:win7:armhf" "com.ubuntu.juju:win7:arm64" "com.ubuntu.juju:win7:ppc64el"] are [{Fri, 26 Jun 2015 10:32:14 +0000 products:1.0 content-download  [] streams/v1/com.ubuntu.juju:released:tools.json [com.ubuntu.juju:12.04:amd64 com.ubuntu.juju:12.10:amd64 com.ubuntu.juju:13.04:amd64 com.ubuntu.juju:13.10:amd64 com.ubuntu.juju:14.04:amd64 com.ubuntu.juju:14.10:amd64 com.ubuntu.juju:15.04:amd64]}]

2015-06-26 11:55:37 TRACE juju.environs.simplestreams simplestreams.go:930 finding products at path "streams/v1/com.ubuntu.juju:released:tools.json"

2015-06-26 11:55:37 TRACE juju.environs.simplestreams simplestreams.go:466 fetchData failed for "https://s3-ap-southeast-2.amazonaws.com/78ac3a374f2f4c498475863a430c9b1f/tools/streams/v1/com.ubuntu.juju:released:tools.json?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAJETX5WILMFNUU5BQ%2F20150626%2Fap-southeast-2%2Fs3%2Faws4_request&X-Amz-Date=20150626T195537Z&X-Amz-Expires=604800&X-Amz-Signature=c9f8ff73c45374f1e07c38f525b007085abd147621c8edad3ff6993b543b5935&X-Amz-SignedHeaders=host": The request signature we calculated does not match the signature you provided. Check your key and signing method.