You probably wouldn't purposely keep your research from a readership that might benefit from it. Yet when many authors sign traditional publication agreements they sign over all their rights - including copyright - to the publisher or journal.
University of Minnesota Libraries maintains a site explaining the copyright basics surrounding
More about Copyright in the UMD Copyright research guide?
RoMEO summarizes and ranks publisher copyright policies according to four color-coded categories. You can search RoMEO by publisher name, journal title, or ISSN.
For information on research funders' mandates and guidelines, see RoMEO's companion database, SHERPA/JULIET.