There are companies out there you can sign up with as your preferred provider who will provide a little money to the property because you are marketing their service for them to your residents. I believe this is both legal and ethical. Those companies actually send reminders to residents whose policies are getting ready to expire and they will alert the manager when someone cancels, does not renew, or gets canceled because they failed to pay their premium.
I see no con in requiring proof of renters insurance. It is a win-win for both the property and the resident. The only con could be at renewal if someone can't provide proof of insurance. Usually when the property embraces this requirement and educates the residents, most buy-in to it and they renew the insurance without fail.