Doesn't that depend how much you pay? You can pay $2.50 to the soft drink company for your verification can, and Twitch receives $0.10 - why would either you or Twitch prefer that to you giving Twitch $0.50?

Because Twitch would rather get the $0.50 and the ten cents from verification.

Also, if they let you pay $0.50 to get rid of the ads, then their ad audience instantaneously turns into "people who do not have $0.50 of disposable income".