Except,
A) The Yeerks are going for a covert invasion - having these stationed everywhere isn't very covert
B) Visser Three can also morph - wouldn't they attack him too?
C) You have to be in your natural form to acquire animals - since the Yeerks think the Animorphs are Andalite bandits, it shouldn't take a robot to spot an Andalite in public. And since they're actually humans, the robots wouldn't notice them anyway whilst they're acquiring, would they?
Forgive me if I've got something wrong - it's been years since I read this book.