I say avoid the store if you want. If you feel the need, I also see no reason not to tell the store owner why you will not be shopping there. As other posters have said, stores won't keep bringing in supplies if they are not selling. My sister, one time, refused to go a certain restaurant as it served shark meat (in a soup, as I recall) -- I don't know if other patrons felt the same way or not but I notice they stopped serving shark just before they went out of business.
I believe it to be true that horse meat is routinely put into pet food. My small animal vet has said that when you read the list of ingredients on the label, the ones identified as just 'meat' are horse meat.
Unfortunately I don't have a solution to keep the thousands of horses that end up in the slaughter plants out of those places. I can only urge people to think responsibly and compassionately when it comes to breeding the backyard mare, keeping the large brood mare band, starting horses too young, or using horses too hard.