I think this should be practiced by every gambling site and, in fact, buy any site where your account can be of some value. It is very rare when normal users want to change their email address, so support would not have much of the additional work to do in the case of implementation of such a practice. In most cases it's hackers who seized control of an account want to change the email associated with it, so this should be made as hard as possible for them to perform.