Wells Fargo Mortgage Problem with paying extra principal, WFM charges interest illegally California

I am trying to pay extra to the principal every month.

All amount below are approximate...

Let say my payment of P & I is $400.00 for this month.
I pay $1,000.00 -- expecting the extra $600 to be applied to the principal.

Well Fargo mortgage system processes the payment in the following way:
first $400 ($330 interest + $70 principal) for this month
next $400 ($328 interest + $72 principal) for next month
and $200 to the principal.

Why would I want to pay them interest that has not accrued yet??
Is this even leagal to charge interest before it accrued?

Let say I have a 15 year loan and I decide to pay the loan off.... are they going to charge the interest for the remainder fo the loan first and then apply to the principal???

Of course when I complain -- someone fixes the problem by applying pament correctly. Why should I even be contacting them about this problem???
I have other mortgage payments that I prepay -- and I have no problems.

Disappointed
San Diego, California
U.S.A.