Best Buy/HSBC have a record of such unauthorized charges, along with problems in "no interest for x months" deals, etc. That is one reason you should never have store cards and accounts, once you establish sufficient credit to have decent unaffiliated credit cards.
You don't want your bank in cahoots with any merchant you use your card with, conniving on how they will divide up your money. Instead, you want your bank on YOUR side in any dispute.
In fact, when you are making a dispute, particularly when you are dealing with an adversary you cannot trust, you do not want to be dealing by phone. Send your dispute via U.S. Mail, certified return receipt requested. Under FCBA, if you send a dispute within 60 days of the statement date showing the disputed charge, they must investigate. If they slipped this through without your authorization, and got past 60 days, your best bet is to go balistic and file complaints with BBB, your state AG, and FTC, noting on your dispute letter "CC: BBB, <xxx> AG, FTC" so they know where you have complained.
What you are pushing on, beyond your FCBA rights, is that HSBC also has a fiduciary responsibility toward you their customer, beyond disputing a simple arms-length third party merchant charge, since they are aware, or should have been aware, of the deceptive trade practices employed by their partners Best Buy and HSBC Insurance Services, hence they were a willing partner in that deception failing to meet their fiduciary duties to protect their customer, you, from fraud when they are aware of it.
