My travel policy says we have to rent a compact and should turn down upgrades due to the potential for increased fuel costs. If Hertz knows about that, or anticipates that kind of rule, it wants the non-corproate travellers in big cars so it doesn't have to swap the (numerous) corporate travellers.

Judging by the very many triumphant, list-your-upgrades type of posts here, I highly doubt such a policy is really that numerous. In some cases people who work for small companies are required to reserve a compact, but they often use a free upgrade coupon to avoid squeezing into an uncomfortable econobox for a long business trip. They say with apparent envy how they wish they could choose a fun car from the Emerald Aisle or Ultimate Choice, but that would require paying for an intermediate, which their employer will not let them do.