It's easy to understand why the theft of items is not allowed. It's harder to understand why the theft of animals is also not allowed. Ship theft is allowed on the basis that there is an easy way to protect against it, by placing a piston. Animal theft is as easy to prevent against, by attaching animals to a leash or keeping them in an enclosed area. It's a little absurd how you can get away with stealing a dreadnought that took some newbie a week to build but you can get banned for days for luring sheep outside of claims. It's a good way to entrap people and get them banned. Furthermore, the neccessity of this rule is really questionable. If it was only on Towny, that's understandable, but this is nations.
Disclaimer: I have never been banned for animal theft and I don't intend to steal animals, I'm only expressing my thoughts on the rule, I think it's a step in the wrong direction, into becoming a babysitting SMP like most servers