Getting Fired: Employer is Requring Me to Voluntarily Resign After Using All FMLA

My question involves labor and employment law for the state of: California

I am currently on FMLA that runs out on 1/20/18 and I was reading the employee handbook today and found this:

Failure to Return to Work Following FMLA Leave
If the employee does not return to work following the conclusion of FMLA leave, the employee will be considered to have voluntarily resigned. COMPANY may recover health insurance premiums that COMPANY paid on behalf of the employee during any unpaid FMLA leave except that COMPANY’S share of such premiums may not be recovered if: (a) the employee fails to return to work because of the employee’s or a family member’s serious health condition or, (b) because of other circumstances beyond the employee’s control. In such cases, COMPANY may require the employee to provide medical certification of the employee’s or the family member’s serious health condition.
For further information or clarification about FMLA leave, please contact the Human Resources Department.

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I'm interested in the bolded section - is this legal?

Thanks so much.

Getting Fired: Employer is Requring Me to Voluntarily Resign After Using All FMLA