This is honestly the first time I’ve heard of this, but it sorta makes a bit of sense I guess considering that some beekeepers spend lots of money to have queens that are bred with certain traits.
Though your typical usual run-of-the-mill swarm off a tree doesnt have a great probability of having one of that particular beekeeper’s queens in it. I guess if he can confirm that it’s his queen (many beekeepers mark their queens with a dot of nontoxic permanent paint to find them easier and remember how old they are) then I’d understand him wanting it back. Otherwise I say he’s just being mean and taking an obviously strong queen from another keeper of the bees.
If someone comes up to a swarm you’re picking up and says that those are their bees, you can rest assured that they are prooooobably just trying to get some easy bees off of you. (Unless you live very close to a large-scale apiary. They send to have massive swarming seasons when they aren’t properly managed, even otherwise they send out maaaaany swarms that they will often want to round up and bring back home asap.)
People really want those bees!!