Is this gross, and do they need stopping? Of course.
Is boycotting mars going to make even the slightest difference? Not in a million years.
Not only does mars probably own more companies than you even realise, including many of the alternatives you're buying thinking you're avoiding them, but even the products you do buy that are coming from a different company altogether, suffer from the exact same background problems (exploitation, oppression, unsustainability, lobbying).
There is good reason for the saying "no ethical consumption under capitalism", and the answer isn't making bare minimum and counterproductive gestures like trying to find some ethical unicorn of a company, it's to abolish capitalism because it requires and encourages all of the unethical practices you're looking to avoid, in order to exist.