Update, 2017 Bluetooth 5.0
In summary, Bluetooth 5.0 comes with a lot of bandwidth, which allows the streaming to two speakers at the same time.
Speaking about the 5.0 Bluetooth in the Samsung S8, AndroidPolice writes:
That extra bandwidth is required for the Galaxy S8's "Dual audio" feature. This allows the phone to stream audio to two Bluetooth headsets or speakers at the same time. You can also adjust the volume for each device independently. It's like sharing an earbud with your friend, except with fewer wires and less bacteria.
I think the bandwidth allows for sharing to two speakers at the time. I am yet to find out if any extra speakers could be added.
But there you have it. Bluetooth now evolved to support bigger bandwidth to support multi-streaming, Natively.
Any thanks? Give to the Bluetooth gods!
Because this resource isn't as old as the current question is, I am sure it is the answer.
The limitation for sending output to multiple connected bluetooth devices is not about a feature. Its about how Bluetooth enabled communicate with each other.
Bluetooth, from day one, was designed for a one-on-one communication over short distances for short periods of times i.e sending an image, short video clip etc.
The use of Bluetooth has evolved and as such support for multicasting is much wanted.
Quoting from this XDA forum post:
The reason why bluetooth doesn't support Multicast is because it doesn't have the bandwith to support High Quality streaming simultaneously no would the chips really having the processing power but that could be fixed, but the main limitation is the lack of available bandwith specified in the 802.11 standard for bluetooth.
... it is clear there's a limitation, and hopefully work is done in that regard to allow support.