Is that not free? Sorry I use twitch purely as a way to do amateur streaming. Apparently I know very little about the viewership features.

On Twitch, "Subscribe" doesn't mean the same thing as on YouTube; it is a $5 monthly donation (except when a streamer has different tiers) that gets you no ads for that streamer and, depending on the streamer, exclusive emotes/chat room access. The typical type of subscribing where you are notified about a Streamer going live is called Following.