I've never had one that got them until Dannyboy's last year. He had that cancer though, so he was sick, and I was attributing it to that. Didn't worry about the tear stains though. There was too much more to worry about.
Everything that I've ever read about it attributed it to some kind of imbalance in the body. I didn't know the tear stain products had an antibiotic in them though until now. That's definitely not something you want a pet on long-term if it's not necessary. If a pet gets really sick then, it's harder to treat an infection. Plus, you're ruining their gut by doing it.