I found the following info online and the website is at the bottom of the post. I think perhaps the officer asked you for fingerprint based RCMP because it was specific to your case only - in general I think the US Embassy will accept RCMP certificates based on name and birthdate. I think they determine whether a fingerprint based on is needed only on a case-to-case basis - dont you think?
Canada
Information about police certificate from Canada can be found
here[18].
They are also called RCMP certificate as police certificates from
Canada are obtained from RCMP (Royal Canadian Mounted Police).
Police certificates are required for consular processing for
those currently living in Canada for 6 months or longer or for
those not currently living in Canada, but who did live in Canada
for more than 1 year.
If you look up the blue pages (government offices) of your phone
book, you will see RCMP phone # under "Canada" section. In
Toronto the number is 416-363-9072
There are two types of police certificates, one is name check
based and other is fingerprint based. Only name check based
police certificates are required for US immigration. Fingerprint
based police certificates are for Canadian immigration.
Other than a valid reason they do not generally do it, just like
that. So you have to tell them you need to do it for "US
Immigration".
The police station issuing the clearance will red stamp an
approval and on the bottom of the form you will see a note;
"details cannot be confirmed as relating to the subject of
inquiry without fingerprint comparison". Do not be alarmed by
this because fingerprints are NOT required. If, during your
consulate interview the consular officer finds your police
clearance has a "hit" or name check problem you may be instructed
to be fingerprinted. In this case the US consular post will
furnish Form FD-258 to be used for fingerprinting. This is the
only form they will authorize for fingerprinting. Not any RCMP
fingerprint form. Anyone who has initiated on their own to secure
RCMP fingerprints is wasting their time. The consular officer has
to witness the issuance of finger printing for authenticity. The
RCMP fingerprint form is used for Canadian immigration or
criminal processing.
http://www.immihelp.com/immigration/police-certificates.html