This would be like if you are a dairy farmer and you notice people who buy cookies usually buy milk, so to make things simple you make an agreement to pay a store 25 cents for every cookie they sell (because you want to incentivize them to sell more cookies and therefore more milk). You couldn't then accuse a customer of fraud when they buy cookies but not milk. They never agreed to always buy milk when they buy cookies, that was just an assumption you made.
Whatever the merits of that argument might be in the general case, using an extension which expressly advertises its function to include falsifying clicks to mislead ad networks makes it hard to make the argument in that context.