Often those who complain a lot are the ones who care the most. They're, in a way, protesting something out of their control. The difference between needless complaints and legit protest is the effort put into exhausting all other options before the complaint arises. In other words, we've done everything we can and this needless injustice still exists, so it's time to let others know it's an issue so those who can change it will change it.
Next time someone is complaining about something at work, first ask, have we exhausted all options within our power? Then, is it worth fighting for? If yes to both, it might be a good idea to start complaining too.