This is the expected behavior. In the current model when sharing on TIBCO Cloud™ Spotfire® with a selected groups of users or publicly (not within the TIBCO Cloud™ Spotfire® team), the "shared to" person only gets READ permissions, because you are viewing the file by browsing to the author's folder. Users are not granted WRITE permission to other users' folders.
Alternatively, you can instead save the file in your own library (this will not effect the shared version from the original author's library).
If you want to share files with your team members, and would like them to have write permissions to these files, you need to save the files in your teams folder, e.g. Library › Teams > YOUR Organization. See -