To be clear, this was the employer refusing to close the email account / set up an out of office, they just wanted all email redirected to them. Nobody said anything about being able to continue using the email.
... this is totally reasonable for an employer to do. For example, sales person at a company. Probably sent/received many emails that are relevant to sales at the company, rather than using a mailing list (I see this a lot). After the sales person leaves the company, the company needs to maintain relationship with the people who worked with the sales person. Keeping the old email open rather than dropping them on the floor makes sense (otherwise your customers will hate you).