I don't know if dispensing cannabis to a pet is harmful, so I choose to err on the side of caution. Is it fair to assume you don't think there is anything harmful whatsoever about 'mary jane'?
Of sound mind, I try to evaluate all the substances I consume. Some I know can be lethal, some I know are healthy, some are legal, some aren't. My pets, irrespective of how I perceive their enjoyment, aren't capable of evaluating some of the things they consume, and the effects those products have on them.
I'm quite certain, though, that it takes a genuine moron to suggest blowing smoke into a pet's face is sensible unless the pet is desperately ill, or terminal. I smoke knowing it's fucking stupid, but unfortunately I love it. My pets aren't aware of the risks involved, but since I am, I don't do it around them. It's one thing to pollute yourself, which everyone is free to do without judgement, but it's another to wilfully pollute a pet in your care. Others here have mentioned administering cannabis products to a pet through vapor, medibles, and oils. I'm happy to listen to arguments in favour of those methods, because there aren't any carcinogens involved, although I would still take a lot of convincing.
I get it though, you think it's cool. You've made your point.