He's still on Youtube, he can continue to speak. Google only ended a partnership with him and is no longer going out of their way to promote him. Google doesn't have an obligation to tie their brand to one that's engaging in unpopular speech, as that would defeat the purpose of a promotional relationship.

Having commercial partnerships with YouTube is not a constitutional right; they have no obligation to keep working with anyone.

Youtube made a business decision not to support someone that does not fit into their corporate culture.

The jokes were done in poor taste, and it made bad PR for Youtube/Google.

Unpopular speech does not need any more protection. It's already afforded the same protections as popular speech.

Google owes a fiduciary responsibility to its investors. Maintaining relations with a fool like that is bad for business, and could expose them to shareholder suit. There is even more than speech, not that Google wields coercive power to prevent speech in the first place.

So an employee who makes offensive, unpopular speech or content should be protected from his bosses firing him, especially when he represents the company? Like it or not, Pewdiepie represents Youtube in a large way and the things he says represents, in many respects, how others see Youtube. Is it an overreaction from Youtube and Maker? Maybe, there are arguments that support it. But it is their right to essentially fire him (his show was canceled, he was removed from his network, and he can't use the same advertisements as some other content creators can) if they don't like his speech (as long as the contract allows it). Just because he's allowed to say whatever he wants and justifies it as a joke doesn't mean he doesn't have consequences for those actions, and I strongly think that he chose to use Nazi and anti-Semetical symbolism for the shock value when he could have conveyed the same message with something less offensive.