Can credit card companies close your accounts without warning? How does it affect your credit score?

I received a letter in the mail from Juniper (iTunes Rewards) stating that they have closed my credit card account due to inactivity. I opened the account about 18 months ago and have never actually used it. Can they close my account just like that without any warning? How should I handle the situation? (Is there anything I can even do?)

Also, I know that closing credit card accounts lowers your credit score. Does anyone know how much mine will be affected?