.mts files are avchd files having metadata which is required by premiere pro to read and interpret those files. When you browse the files through media browser, premiere pro reads the folder structure and the metadata of those files and imports them accordingly. If you drag and drop it directly in the project panel the folder structure of the files are damaged and premiere pro does not interprets them correctly because of missing metadata. CS6, was not so strict about the folder structure and sometime interprets the files correctly, but CC is little more strict aboit the folder structures and metadata files. For more information about folder structure of .mts files you can check help file of premiere pro.
Metadata contained in folder http://forums.adobe.com/thread/1015001?tstart=0
Thanks so much for your answer - that cleared up a lot about MTS support. However, that was the issue I experienced previously. Once I imported the MTS files through Media Browser, Premiere began working without a hitch.
However, when I came back to work on the project on a later date, it gave me the pop-up you see in the image above. Do all files (.PSD, .JPG, .MP3) need to be imported through Media Browser? I only listed those file extensions because those were the only other file types used in this particular project.
This foldet structure and metadata concept applies only for the clips shot by camera and giving a proper folder structure like .mts, .mxf and more so .jpg, mp3 and psd can be imported directly.