I think it's more like the non-keyworded default definitions of class
methods, for the same reasons; the default definition has to potentially be
valid for all instances of the class​.
It's the difference between
class Applicative m => Monad m where
return :: a -> m a
return = pure -- always valid, but can be overridden in instance
declarations
and
class Fuctor f => Applicative f where
(<*>) :: f (a -> b) -> f a -> f b
default (<*>) :: Monad f => f (a -> b) -> f a -> f b
(<*>) = ap -- only valid if matches the type signature above
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