You may want to increase logging for org.alfresco.rest.framework.webscripts.ResourceWebScriptGet to check wether the fields parameter is properly recognised. When set to debug you should see an output line about "Filtering using the following properties" when the fields parameter is correctly recognised.
I see no inherent bug in the source code. Are you sure you are using the latest stable version of Alfresco for your release?
because the "title" is in the sub-object "site" of the entry. You can see this when using fields=site You would have to figure out how to address the sub field - my naive approach would have been fields=site.title but this doesn't seem to work either...