It's wrong to throw stuff on at somebody, or if it's likely to cause injury. That wasn't the case at all with the tennis ball protest. You have a very dispassionate view of sports. To you it's no different from going to a cinema or going bowling, and if you don't like the "product" you just go somewhere else or don't go at all. The fans want to go to all the games. They just don't like outside interference - interference their club might be totally against - making supporting their club more expensive, more inconvenient, and altogether a less enjoyable experience.