The fuel issue arises during the event, not during evacuation. There's almost always fuel for evacuation.
The evacuation routes are the same routes used to come into or back into a region. Every town has evacuation routes. Requiring the standby power along these routes assures each town has fuel available post-disaster.
Fuel is needed for rescue & recovery, much of which is not done by local authorities. You have not only the locals trying to get ice, food, lumber, plastic, etc. but you also have relief agencies and volunteers coming in. Fuel shortages are another accelerator to civil unrest. Having fuel post-disaster is a necessity. I went through Charley, Francis, Ivan, Jean, Rita, Katrina & Wilma. It can and often does take several weeks to restore utility power.