Per my understanding of fuser, if /dev/sdb1 is mounted on /mnt, then both 

    "fuser -m /mnt" and 
    "fuser -m /dev/sdb1" 

should give the same output.

This interpretation of the fuser command line is corroborated by an upstream
bug related to the functionality and the fact it was fixed for psmisc v22.20

http://sourceforge.net/p/psmisc/bugs/53/

Apparently there is a regression in v22.21:

Neither the unpatched psmisc v22.21 source nor the patched version in the
openSUSE 13.2 release function that way, but they don't function like each
other either.

The unpatched source creates a fuser binary that complains /dev/sdb1 is not a
mount point:

# with vanilla upstream source
> fuser -m /dev/sdb1
Specified filename /dev/sdb1 is not a mountpoint.

That is literally true, but the man page for fuser says a block device can be
passed as a value for the -m argument and the mounted filesystem will in turn
be queried.  Thus the vanilla source from upstream has a bug.

I have added a comment to <http://sourceforge.net/p/psmisc/bugs/53/> that it
should be re-opened.

==
If testing the fuser binary provided with openSUSE 13.2 a different wrong
result is provided:

# with openSUSE patched source
> fuser -m /dev/sdb1
/dev/sdb1:            1150rce  1152rce  1206rce  1207rce  1209rce
1312rce  1353rce  1355rce  1358rce  1363rce  1376rce  1377rce  1379rce
 1385rce  1387rce  1391rce  1394rce  1402rce  1409rce  1411rce
1448rce  1461rce  1469rce  1470rce  1475rce  1512rce  1631rce  1635rce
 1644rce  1658rce  1661rce  1664rce  1665rce  1680rce  1685rce
1728rce  1781rce  1791rce  1801rce  1807rce  1810rce  1811rce  1815rce
 1835rce  1839rce  1897rce  2355rce  2378rce 11758rce 18751rce
18849rce 19093rce 19106rce 19120rce 32545rce

With the openSUSE patched source, the binary seems to treat /dev/sdb1 as a
standard file and reports all open files on the /dev filesystem.

That is much worse than the vanilla source does which is a simple failure.