It makes sense to allow collaboration in some contexts (homework) while forbidding it in other contexts (test taking). This way, students are rewarded for working together yet are still have an incentive to make sure they aren't simply letting their partners do the work at the expense of their own understanding (intentionally or not).

The cheating happened in a testing context. Other parts of the class would have emphasized the collaborative aspect of learning, and I'm sure this contributed to the widespread consensus that sharing answers was OK, despite written instructions to the contrary.