The reason why we hate it is because it creates confusion about what a "vegan" is. Nobody cares if you want to eat an entirely plant-based diet except for bacon, but calling it "vegan + bacon" is misleading. It would be like if a Christian went around calling themselves an "atheist" because they disbelieve in all gods except for the one true God in the Bible. That isn't how the term "atheist" is commonly used and understood.

Vegans already have to deal with people who think that vegan is the same as vegetarian, or that veganism has something to do with health (it doesn't), or that it means you're also gluten-free, etc.

I recently ordered a sandwich labeled "vegan" at a restaurant. Mayonnaise was listed as one of the ingredients. Of course, there is vegan mayo, but I thought I better ask just to make sure. Maybe these people don't know what "vegan" is. Sure enough, the mayo was made in-house, and it contained eggs! I was trying to explain to the lady that eggs weren't vegan and her menu was misleading people, but her English wasn't very good and she couldn't understand me. So I finally just gave up and ordered the sandwich without mayo.