They do get taxed on it, the same way that you are required to report earnings on tips. It is a matter of two things:
1. The individuals reporting the earnings at tax time
2. the size of the bank deposits. Any deposits over $10k automatically get reported to the IRS
If they lie when they self-report at tax time, the any bank deposits on record from their banks will get them caught. They will be fined at first, and if they fail to pay the fines, jailed. (The IRS would rather have its money than see you go to jail)
Depending on how the tips are paid out by the streaming service will impact how they report, for example, if there tips are paid out monthly then many will likely break the $10k threshold upon deposit and end up reporting and paying the taxes on it.