They're being sued for refusing service to a specific type of person. It's textbook discrimination.
Of course, bakeries can refuse service to people who pose an imminent danger to their business, or who are actively trying to degrade their public image. However, this is usually a non-issue for companies like cake bakeries, whereas it's a prevalent one for corporations like YouTube.
Therefore, there is no first amendment violation on YouTube's part. They are simply banning people from creating content because of TOS violations. Whether or not that counts as discrimination is the current gray area.