If I do :
$ scp -l 1024 remote1:/path/to/file remote2:/path/to/destination
I can see, using nload on remote1, that the bandwidth used by scp isn't limited to 1024 kbits/s like I thought.
I found a workaround :
$ ssh remote1 'scp -l 1024 /path/to/file remote2:/path/to/destination'
But for the sake of knowledge, is it normal behavior that bandwidth limiting in my first command doesn't work ?