Because viper-mode only covers vi features, not vim features. Once you start using text objects[1], its hard to go back to normal vi.

[1] Like, if your curser is anywhere inside a set of parens you can type 'yap' to copy the entire paren block or 'yip' to just copy the stuff inside the parens. And unlike 'f0y%' it always just does the right thing with nested parens.