Why just Flash though? I thought Flash was on its way out. Other than that, interesting move. Looks like there is increased interest these days toward live streaming.(Youtube live was announced recently as well)
Flash Media Server means Adobe software running on the server, so users would either need to arrange the license themselves and pay for a server (e.g. using EC2) or have a specific service offered by Amazon (what they've just announced).
Anyone who wants to serve content without flash doesn't need Flash Media Server, so they can do it without needing this service.
It is $5/month, plus the other Amazon charges.
http://www.wowzamedia.com/ec2.php