I thought I'd read the original data plans, and IIRC they all
do provide for discretionary limits, even the oldest ones. If you break the limit every month, and you do it heavily every month, i.e. streaming 10GB of multimedia, the discretion ends. If you toe the line, maybe hit 6GB every third month, 4GB every other month, they look at the averages and give you a pass.
But they still are looking, and while they'll never tell you what they are using as the basis for decisions, this is all just a line or two of programming code for them. It kicks out a victim list every month, and folks get invited to pay or go.
Just for laughs, try using 10Gb every month for six months in a row, see if they don't give you a call. <G>
In Lily Tomlin's classic words "Because we're the phone
company and we don't have to give a damn."