Why does the bank want my social security number for opening a checking account?

I went to open a checking account today at a local bank and they insisted on having my social security number in order to open the account. I have a SS#, I also have an EIN (Employer Identification Number). I had no problem giving them my EIN, but they would accept it without my SS#. I ended up walking out. I... show more I went to open a checking account today at a local bank and they insisted on having my social security number in order to open the account. I have a SS#, I also have an EIN (Employer Identification Number). I had no problem giving them my EIN, but they would accept it without my SS#. I ended up walking out. I don't feel the need to give a bank my SS# unless I am borrowing from them. I was not. I was merely trying to open a checking account. Do they really need it?
Update: Why is it required? It's a non-interest bearing checking account.
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