I've Been Sold

I'm hopping mad, my credit card company informed me that they sold my account to another company, which promptly raised my interest rate 5%. How can this be legal? I have the option of refusing the new interest rate, in which case they will close my account and allow me to pay off the account at the old interest rate, with the usual minimum monthly payments. I only have one credit card, so if I did that, I'd have no credit cards at all.