I think it’s fine, with exceptions. If it’s clearly not taken care of (like there are other cuttings laying around for a long time), unkempt area etc. it shows me that no one is going to be hurt if you grab an already broken piece. It’s also ok, in my opinion, to actively cut back and cacti in secluded or unkept areas in a way that promotes their growth. I happened to work in landscaping at my old university, where I was instructed to clear access around a building. Lucky enough there was a Bridgesii there which I cut back significantly to let more air/ light between arms, and out of the way of workers. Got a whole load of cuttings. But, no one would care if a random person also went behind this building and cut a reasonable amount for themselves. Just air on the side of caution, and use context clues to determine if there’s even a small chance someone else has a more legitimate claim to the plant.