It is illegal to take anything from private land without the permission of the landowner. Removing artifacts from sites disrupts the archaeological record because the artifact is removed from its context. Collecting sites is a huge problem in Utah. Some archaeological sites are no longer visible because all of the artifacts have been collected. No, archaeologists deal only with material left by human beings.
Paleontologists are the scientists who study dinosaurs and fossils.
I get this question almost every time I visit a school. I use this as an opportunity to talk about other things archaeologists are interested in besides the artifacts. I use an example of pinyon pine charcoal from fire-pits 6000 years old in Colorado where currently there are no pinyon pine trees. Sometimes I talk about coprolite studies as a way to talk about other things of great interest and importance besides artifacts. I often show a slide of an archaeologist taking a soil sample then talking about the tiny seeds and such recovered from the soil and what that can teach us.