Under California law all IP produced by an employee during their employment, that is within the domain of the employer's work product and regardless of where that work was produced, is by default owned by the employer. All IP produced by a independent contractor is owned by the independent contractor, you pay for their time not for their work product.

'Work for hire' clauses are used to simply and effectively move an independent contractor into the same category as employee with respects to the IP they produce. It does not do the same with respect to the independent contractor's compensation, or rights as employees.

Generally the 'work for hire' clause itself is entirely inflexible, it is other areas of the agreement that must be addressed to protect payment. Hollywood, for example, is run almost entirely under work for hire agreements and could not function without it.