The seller didn't said they pay for their electricity. If they live in a dorm, or charge the phones in their workplace or school, there are lot of illegal ways to get free electricity. It's easier to conceal a regular phone than an antminer
In a dorm its legal but may be against your contract.
At school/work you probably aren't officially allowed to charge your devices, so it's theft.
Even if you are allowed to charge your phone at work, they technically may have to meter it and tax it as additional benefits depending on your country.
The antiwork subreddit is full of.. dubious.. stories. That article hasn't verified any of the claims and it's well known that sub is (was) compromised of people who got off on manufactured outrage.