Rather, they charge you on a monthly basis, according to your highest hourly retrieval rate from any single hour applied to the whole month (standardized at 720 hours). So if your peak hourly retrieval in a month was 10 GB/hr above your pro-rated free quota for that hour, you're charged for 7200 GB of retrieval overage, or $72, even if you retrieved nowhere near 7 TB of data. For companies with relatively constant retrieval this doesn't matter, but for small users, someone who has a large overage one day will be charged as if they were incurring the same overage continually all month.
To put it concretely, if you store 200GB of photos and then do a bulk retrieval, you aren't charged $1.90 ((200-10) x 0.01). Rather, the xfer is counted as 4 hours, so you have a peak transfer of 50 GB/hr. Your free daily retrieval is allocated to the hours in the day you retrieved data, so in this case 10/4 = 2.5 GB/hr free. So your overage is 47.5 GB/hr. Multiply by 720 and $0.01, and the fee for retrieving 200GB of photos is $342. Plus $22.88 in outgoing AWS bandwidth.
Or at least, I think that's what their pricing page says. Maybe someone else can better interpret this explanation: http://aws.amazon.com/glacier/faqs/#How_will_I_be_charged_wh...