The library provides access to a number of databases that provide the full text of journal articles. Use of these databases is governed by licensing agreements between the library and provider of the databases.
Since the library pays for access to these databases, professors can print copies of full-text articles to distribute in class. This is an educational use of copyrighted material, and the students each have access to the material through the library.
For more common scenarios and how to handle them in compliance to the law, please visit this page from California State University at Long Beach.
Visit this page from Reynolds Community College to learn about linking database articles. Remember, the best practice is to link using persistent URLs or with DOIs.