I AGREE!!You're not taking food out of children's mouths doing what you're doing.
But if EVERYONE did it, consumption would exceed what's being produced by restaurants, and they'd have to increase production of vegetable oil to use for fuel. And doing so would result in more vegetables being used for energy instead of feeding the cute little starving children in El Salvador or wherever...
And demand would also drive up the cost of vegetable oil, making it pointless.