[It was probably on some sort of dare from one of his crewmates. Law's generally a no-fun-allowed sort of person but when it comes to body horror he's infinitely more willing to participate.]

... I'll take the food but I don't know how exactly you expect to test it short of me cutting myself open to examine my stomach contents.
[Which is something he could do, but won't because even with his weird magic methods, it's difficult to perform surgery on yourself.]