I think the word "owe" has a loaded connotation. We're rarely owed anything in our lives, so no, therapists don't owe clients explanations when they cancel. I would consider it a professional courtesy if they offered something, but it may not always be appropriate.

I know that I'm lucky in that I've never had a therapist last minute cancel on me. My old therapist liked to reschedule me, though, and never offered an explanation why. It was problematic once, but there was a lot more going on than a rescheduled appointment.