I agree that Person B's situation feels unfair compared to Person A's. However:
a) It's just one of many ways that employees can negotiate more favourable contract terms. They can also negotiate more annual leave, fewer hours, smaller notice periods etc... and with proper management it shouldn't cause issues.
b) People who work regularly in the office have perks like their own permanent desk and PC.
c) The cause of this unfavourable situation is the 3 day / week policy put in place by senior management in the first place. Imagine cutting the carpark in half to save costs, and then removing the bike racks as well so that the car drivers don't feel as hard done by. It comes across as spiteful.
As for the "added work of managing", I'm not sure I follow. I previously managed a couple of people, one of whom came in 2-3x per week and the other was almost completely remote but came in occasionally. Once things are set up so that meetings or calls can be done remotely or in-person on the fly, this isn't much of an obstacle. Perhaps it becomes more of an obstacle with larger teams, but that isn't something I hear from other managers in my company. It is tricky to run meetings with half the people in the room and half remote - but this isn't solved by forcing remote workers to stay remote (if anything it makes it worse).