If we all did this, students would get a clearer message about what they can and cannot do. One new problem facing many faculty is material that students find on the Internet. You really need to be cautious here. Because so much of what is available is pure junk, students need help in discovering what is good and what is not good. And, should you give an assignment which causes students to find materials on the Internet, students have a right to be provided with guidelines for what will be acceptable and what will not be acceptable in completing that assignment.
Attribution is important here, also.
You may want to share with your students one or more examples of how to cite materials they have found on the Internet. Because this is so new, many of the style manuals have not yet included this type of citation.