Comment by mistermegabyte

They're not burning drive hours, they're burning on-duty hours which are also limited. You are limited to 14 hours of on-duty time. You are also limited to 11 hours of actual driving time out of those 14 hours. So, I guess if you are delayed more than 3 hours (minus breaks) you are kinda eating into drive hours because of the on-duty limit of 14.