When pasting a line with a gradient stroke it gets converted into a path
When copy and pasting a line (with a line gradient) Illustrator converts the line to an outline path. See video.
                2
                 votes
              
            
- 
      
A related problem:
Pasting an object with perceptual gradient applied expands it and converts into an image
http://illustrator.uservoice.com/forums/601447/suggestions/50652335