I would hardly be surprised if doing it this way saves ATT millions of dollars. The default browser for Windows XP is IE6 and a large percentage of new ATT internet customers are highly likely to use whatever browser is on their system, and even a small fraction of ATT's customers is millions of people.
Also keep in mind that this default behavior was almost certainly implemented several years ago when the use of IE6 was even greater than it is today in order to avoid lengthy technical support phone conversations with people like your grandparents when they purchased their first computer.
I'll add that it costs ATT very little to only directly support IE - sure a few calls from Firefox users escalate but that is probably offset by the fact that as a market segment those users are more likely to be technically savvy.
ATT knows its market segment in a way that most startups would do well to emulate - and recognizing how their setup procedure goes to their bottom line is a valuable lesson about focusing on MVP rather than perfect code.