You should ask, and check your local laws. Ontario has the Ontario Bee Act, which among other things states:
Right of owner to pursue and recover swarm
3. (1) Subject to subsections (2), (3) and (4), where a swarm of bees leaves a hive, the owner of the swarm may enter upon the premises of any person and recover the swarm. R.S.O. 1990, c. B.6, s. 3 (1).
Where owner declines to pursue swarm
(2) Where the owner of a swarm of bees that leaves its hive declines to pursue it and another person takes up the pursuit, such other person is subrogated to all the rights of the owner in respect of the swarm. R.S.O. 1990, c. B.6, s. 3 (2).
Owner of premises to be notified
(3) Where the right to recover a swarm of bees is claimed under subsection (1) or (2), the person claiming the swarm shall notify the owner of the premises on which the swarm has settled before entering the premises and shall compensate the owner for any damage to the premises caused by the entry. R.S.O. 1990, c. B.6, s. 3 (3).
When right of property in swarm lost
(4) Where a swarm of bees leaves a hive and settles in an occupied hive owned by a person other than the owner of the swarm, the owner of the swarm loses all right of property in the swarm. R.S.O. 1990, c. B.6, s. 3 (4).
In Ontario, the swarm belongs to the original owner until/unless they give up ownership, or the swarm settles in an occupied hive owned by someone else (I didn't know that could happen). Without their permission, if you setup swarm traps (in Ontario), technically those bees don't belong to you unless they came from a feral hive. (How do you prove/disprove that?!?) Oh well.
I'd talk to them and try to get their permission. Regardless of the laws where you live, it seems the decent thing to do.