If the business model can't be made to work when they have to pay payroll taxes and provide benefits, then shutting down is the only reasonable thing to do. I expect it's both: they cannot or do not want to change their business model to account for the new reality, and are also hoping that a full shutdown will make voters angry enough to get AB5 repealed.

(I'm not convinced that's actually the case, though, but I don't know the math around this, so it could be.)