Why, at Starbucks, is my MAC address provided to Starbucks when I accept the terms of service?

recently I overheard by chance (I wasn't asking) that when I accept the terms of service in order to use the free Wifi at Starbucks, they actually perform an HTTP operation that sends my MAC address in the URL to the service that Starbucks has contracted with. Why do they do that? Are they actually recording all of my traffic? If so that is a serious privacy violation!

Also, why would the Javascript running in the pop-up have access to the MAC address of my computer?