Betting on a horse once to get some money because you actually need the money? I don't see a moral dilemma here.
Betting on the horse you know will win repeatedly just to abuse the knowledge you have and get rich? ... That's a problem. Also, people would start to get suspicious, so that's probably not what you'd do anyway.
Like that guy said to spiderman once - with great power comes great responsibility. Or great knowledge on horse race winners, or great science, or... fill in the blanks there.