Legal Question in Landlord & Tenant Law in California

You are not being penalized. It's really very simple. You owe rent until the legal end of your rental period. On a month to month tenancy that has lasted a year or more, that legal end is 60 days after the landlord gives notice the tenancy will be terminated, or 30 days after the tenant gives notice the tenancy will be terminated. If the landlord gives a 60-day notice, the tenant is free anytime in the first 30 of those 60 days to give a 30 day notice to make the termination date earlier. But if the landlord gives a 60-day notice and the tenant gives no other notice, the rental period for which rent is owed doesn't end until 60 days have passed.

You also ask how you were supposed to know if it was not disclosed in their notice. It is not their job to tell you the law. Also turn the question around. How were they supposed to know you were going to move out early, so they could re-rent the apartment and not lose rent, if you didn't give them any notice? They reasonably expected rent for 60 days and then they would have to get a new tenant or lose rent. If you don't give them notice you will be leaving earlier, you deprive them of the opportunity to replace the lost rental income for days 35 through 60. That can be a serious hardship on a landlord who still has to pay the mortgage on the building, property taxes, etc.