在mount的man手册中有说明：
The non-superuser mounts.
Normally, only the superuser can mount filesystems. However, when fstab contains the user option on a line, anybody can mount the corresponding filesystem.
Thus, given a line
/dev/cdrom /cd iso9660 ro,user,noauto,unhide
any user can mount the iso9660 filesystem found on an inserted CDROM using the command
mount /dev/cdrom
or
mount /cd
For more details, see fstab(5). Only the user that mounted a filesystem can unmount it again. If any user should be able to unmount it, then use users instead of user in the fstab line. The owner option is similar
to the user option, with the restriction that the user must be the owner of the special file. This may be useful e.g. for /dev/fd if a login script makes the console user owner of this device. The group option is
similar, with the restriction that the user must be member of the group of the special file.
FILESYSTEM-INDEPENDENT MOUNT OPTIONS
nouser Forbid an ordinary user to mount the filesystem. This is the default; it does not imply any other options.
users Allow any user to mount and to unmount the filesystem, even when some other ordinary user mounted it. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the
option line users,exec,dev,suid).
为什么mount命令仅供root使用
If you mount under /usr or /bin or /sbin etc. a directory containing malicious files, the system might fail.
And this is a design in the operating systems like Unix. If you are a user, you don't have the permission. The systems should never go too permissive.
If you still want to allow users other than root to mount directories you desire, please hard-code /etc/fstab so that the system will know and mount them when users specify the commands.