To us, in order for us to care, in order for these characters to be protagonists, they must be disposable and put themselves in harm's way for a lofty cause. We empathize with them because they are underdogs who set out on nearly impossible missions from which they know they may not return in order to serve the greater good. They are expendable yet step up and get the job done because it's just that important even though it's assumed they will die in the process. That makes them heroes.
However, with a title like 'The Indestructibles' it's safe to assume these men are invulnerable. They forcefully seize their objectives, almost without effort no matter how difficult or dangerous it may be, as if success were a foregone conclusion, and swagger away in the end. The notion that something might go wrong or that one of them might get hurt or killed is distasteful, perhaps even alien. So, that they willingly embark on dangerous missions yet are never really at risk of injury or death makes them heroes?
The choices of titles for this movie and how Sly and his crew are perceived says a lot about the differences in our cultures.