Because the side effects of my decisions matter? If the 'identity' of a company is providing good products and doing something negative, then by buying their p

Because the side effects of my decisions matter? If the 'identity' of a company is providing good products and doing something negative, then by buying their products I'm helping them do the negative thing. By buying Apple products I'm, for example, supporting needless patent aggression. I may still choose to buy from them anyway because I find their products so good it's worth the hit, but I don't see any sense in ignoring a company's wider effect upon society - by doing so you're removing any incentive for them to behave positively.