Too bad incendiarydan isn’t around anymore! and that I seem to have come upon your question a year too late. But I’ll try to tackle this large question for you anyway, and relating to both fallen/unattached items and still-growing things:
Legality – it always pays to ask if possible, but really it depends what you’re taking and where you’re taking it from. Rural roadsides are treated as public property in many places, but some things are quite valuable – I absolutely wouldn’t want to be seen even just casually studying the ground on what might possibly be someone else’s property without permission during morel season, for instance. Or, say I saw a nice butternut tree in a neighbor’s yard; that’d be another to ask about. Apple trees and grape vines on the edges of property, on the other hand, I usually feel free to collect from, especially if there’s already a lot of obviously uncared-for fruit fallen around the ground there. Cattails, parsnips, lamb’s quarters, sumac, hickory nuts, black walnuts, and many others are so common that I can’t imagine anyone objecting to their harvest. I harvest these (sustainably) pretty much wherever I see them. And some stuff like burdock is an invasive devil plant (somewhat redeemed by the delicious fries you can make from the roots) that poisons cattle and I harvest with glee (and much effort) as often as possible. Always be respectful and don’t harvest fruther into the land from the roadside, despite the temptation.
Other stuff falls more under the “ethical” harvest part of your question, like sarsaparilla, goldthread, milkweed, hickory bark, and others – especially plants from which the harvest is a single underground storage organ such as roots, corms, or bulbs – which are more susceptible to overharvest. Also, some plants grow more abundantly in some areas and less in others, and it is up to the forager to make the responsible call. Never take too much bark from a single tree, and try to only take pieces that are about to be shed. It’s best to be circumspect when collecting rhizomes, because a single rhizome can give rise to several mature stalks. Another to watch out for is edible biennial roots – harvesting these kills the entire plant, so never take more than half in a patch. You can take about a third of the leafy greens from perennials without harming the plant. Annual greens are prolific and can be collected as much as you like. Shoots of perennials should be collected once per season from each colony. Fruits, berries, seeds and nuts are the gift of the plant and are meant to be harvested; these are produced in vastly larger quantities than are required to perpetuate the next generation. These are, however, important food sources for wildlife.
The following is a paragraph from Sam Thayer’s incredible book “The Forager’s Harvest,” which is where I got most of the above info (if you’re into foraging I highly recommend it; it’s one of the best guides out there):
“There are a few things you can do to increase your chances of securing permission. The first time you talk to a landowner, ask permission to harvest a specific plant that can be seen from the road; make it something like elderberries or butternuts that the landowner is likely to have heard of before. Offer to share your harvest with him (don’t worry; he won’t want any). If the landowner was kind and the property seemed like a promising one that you’d like to return to, bring a gift of some foraged product, such as a jar of jam or jelly, as a thank you at a later date. After feeling assured that foraging really is a hobby of yours and that you’re not up to anything else, the landowner will trust you more.”