You forgot that they still count data use over Microcell 3G against your data allowance even though that data is being routed over your broadband connection.
Well, it is also routed over AT&T's data network. It's not like WiFi - when using 3G over a microcell, you get one of AT&T's public IP addresses for your data.
But it doesn't matter, really. The 3G data over the microcell is so crappy you wouldn't really want to use it for that anyway if you had a choice.