It bothers me that in the U.S., we extend that courtesy to pets who are suffering from terminal issues. But we expect loved ones to hang on and suffer for no real reason other then the vague notion that the imaginary sky man would disapprove.
My grandma passed away 2 months shy of her 101st birthday. I visited her a few weeks before she passed, she was gaunt, skeletal, couldn't see us and was reacting to hallucination caused by their body slowly shutting down. She didn't even know my Mom and I were even there, and when we told her her daughter was there to see her, she said "No, I don't believe it" while staring blanking into the corner of the room. She wasn't suffering from dementia, it was cancer that came back which was killing her. What reason would we not allow a loved pet to suffer though that, but a blood relative, hell yeah, let them lay and suffer for weeks, months, years.
I don't have any grand ideas on how to prevent abuse, I just think it's humane to not let a thinking being suffer needlessly.