"If I refuse to sell you groceries, you might just starve to death."

--The only way such a situation can arise is if the very same government you want to boss those corporations around has been been taken over (or established in the first place) by a cabal intending to use monetary and legal policy to acquire an absolute monopoly over food.

They're getting away with it because you keeping demanding that (their, not your) government swing a big stick.