Cruel is a bit of a strong word. It cannot be cruel to keep them safe, can it? They can also be happy too. My two indoor cats are happy with being inside. My eldest (Maine Coon) loves looking outside & has been with me in the garden when I put washing out but that's it, she's fine with that yet the little one (Birman) is terrified of the outside - he won't go past a door frame even if I am sat outside anyway.