Figure it out, banks!

So, I've yet to make a payment on my car because the payments are not due until the 15th, but I wanted to see what happened if I put $10 in the account via online transfer. In reality, it's all supposed to go to the principal but I look and the transactions shows that like $4.50 went to interest. I called the bank and they said that I had to PHYSICALLY come in and do a "principal only" payment. However, IF on my due date I pay more than the normal payment, that will go to the princpal. I'm sure there's some dandy reason why what I want to do can't happen (and I'm sure neither D.O., B.K., NOR B.N. will fill me in on it so that I can remain insane), but it makes no logical sense to me. This means that we can hunt via internet but can't figure out how to allocate money transfers? Seriously? I'm glad my experiment only cost me $10 instead of more.