On my last bill I was charged 5$ for US data usage, however when i look at what exactly I was charged with it has be downloading 0.00 MB. So I was charged for just bringing my phone into the states? I had it on airplane mode the whole time.
Make sure to find your options to make sure your device does not ever use roaming data. This will prevent it from happening in the future. I went to the US earlier this year and had no charges for roaming as I made sure all data and roaming data was disabled and only used wifi when available.