When we lived in town we had an extensive garden that attracted visitors (hordes of visitors!). Some asked for cuttings or seed, but no one took anything without asking, except two women. They knocked on my door, asked if they could visit the gardens, and I went out with them in case they had any questions. One kept asking about this and that, and I noticed that she managed to keep my back to my potting shed, where I had about 60 rose cuttings rooted. I finally turned around and saw her companion putting 4" pots of little roses in her very large handbag. I made her put them back, escorted them off our property, and told them to never come back!
No, I wouldn't take cuttings, not even from public property. Seeds from roadside plants, yes I would take a few, but never strip an area.