I'll try to explain this with an example. Let's say you have a box with a user called weebl. all files on the box are readable to anonymous users, and weebl has a home-dir in /home/weebl. now, if weebl logs in locally (or through ssh), he will be able to read all files, obviously. and write to his home-dir. if you, as an administrator, make his home-dir available with samba (for example) you can control what weebl can and can not access by tweaking samba; you can make him able to access files in his home-dir only, or read all files, etcetera. this is what they mean with "control what the user has access to". using sshfs though, is comparable to what a user can access when uses scp, or ssh even. for example, he could do:
sshfs weebl@server:/ /path/to/mount/point
There is no way to prevent this from happening with sshfs.