as Bigbrother said, the engine cover is paid for by marlboro, so another sponsor can't take that place for NT races, unless an agreement is made where Marlboro get a refund for their sponsorship, but changing sponsors might constitute a radical livery change, which is not allowed. a car has to be generally the same through the year.
also, those who follow F1 regularly will know that there should be a 'Marlboro' logo on the engine cover when they see the white engine cover at an NT race, so the advertising is still effective. the red and white still suggests Marlboro, even if the letters of the word are not there.
for an NT race, to remove this suggestive advertising of marlboro, they should have a completely red engine cover, without the white.
RIP Jules, never to be forgotten. #KeepFightingMichael