I believe the share permissions aren't tied to the files / folders themselves, they're kinda stored in the registry, so most file copy utilities can't get those.
There are several "how to" guides online detailing how to "backup share permissions" by exporting the relevant registry keys, then importing them into another server, but most fail to mention that for that to work the new server has to be essentially identical to the old in terms of where the files are and what partitions they live on. Still, it's better than nothing.
FYI - For non scripted / scheduled backup copy jobs, robocopy is not ideal. For the big one off copy job, find yourself a nice GUI based utility. Many of them don't even have to be installed and are every bit as powerful as robocopy.
Here are some different tools, with a focus on speed, that will retain all the NTFS permissions, timestamps, ownership, etc, as an example you have SecureCopy, Gs RichCopy 360, Copyhandler and Carbonite