Looks like Do At&T is measuring some other aDSL user...
You might try an experiment and expand the power down hours.. see if you can determine the actual cutoff times(GMT,EST,CST,PST,if there are any).. Do some measured experiments.. like download exactly 100 MB in the middle of shutoff period.. 200MB, etc..
Document everything..
If AT&T starts charging you extra..
File a complaint with the FTC after they've received a fair number of complaints...
The FTC will sue "AT&T" for "unfair or deceptive trade practices"