The feature has been in beta for weeks, but should be widely available to both affiliates and partners. They're enabled by default, but can be turned off.

There's no mystery as to why Twitch would do this. Sub and bit trains contribute directly to the service's bottom line -- it wants to encourage that behavior as much as possible. At the same time, it's hard to see many users objecting to Twitch's move. These frenzies already happen organically, and you might as well get something for your effort besides the satisfaction of supporting a worthy broadcaster or a charity.