A banned player might be able to log on the server using a VPN or some other form of a masking software, or access it from their other device, if they have one. Then they can pretend that they are someone else and use those assets that they have been so "generous" to "give away". Not saying that every banned player would do this, but they can do this. In fact, some have done this.
Also, don't call any people evil right away. Remember to use Hanlon's razor first. However, devs are an exception. Devs are not people.