Greene and Baja-
Yep, I do think about the "if everyone did it" concept. This is why, for example, I do not take plants while hiking. But the shrub I wanted a cutting of was 8 feet tall, and at least 10 feet wide, and far from any main pathway. It had "a zillion" branches. I had also looked at the plants in the greenhouse, which are for sale, and propagated by volunteers from cuttings or seeds from the plants in the Arboretum. They did not have that particular plant.
It seems to me that Arboretums might be missing out on an opportunity for a bit of income. I would have been delighted to pay for my cutting!
Still thinking about the issues, just because I don't agree with a particular law does not mean I do not have to obey it, or pay the consequences.