Why do some universities refuse to issue duplicate diplomas?
It comes down to what the theory is about what a diploma is.  Some consider it a piece of paper that you can hang up and not an explicit representation, some consider it a legal document.  (Having degrees and working in Higher Ed for 20 years, often with various policies including those of Registrar's offices gives me some insight.)

Reasons for the policy: 
These policies are based on legal theory or precedence regarding identify theft and legal documentation.  Schools with a higher 'reputation' or a better 'ranking' will have more restrictive policies to protect themselves. 

Queens U, above, states "Each diploma is considered a one-of-a-kind legal document."  

If you give/sell a copy to someone to attempt to represent themselves as having a diploma, then it might be fraud. 

What is the difference between having two originals, and simply saying 'I lost my original, can I buy another?'  --  It could be quantity. If you can go to a web site, pay $40 each and buy 100, then sell them all on Ebay with a "custom name" for $500, that's fraud.  The schools are attempting to protect themselves in a few ways. 
Here's a sample of a fake diploma for sale:
http://washingtondc.craigslist.org/nva/fns/5668595297.html 

Here's a list of real diplomas for sale:
http://www.ebay.com/sch/i.html?_from=R40&amp;_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xdiploma+degree.TRS0&amp;_nkw=diploma+degree&amp;_sacat=0 

People lie on resume's and they lie with diplomas.  

Reasons why such restrictive policies are nonsense: 
IANAL, though, legal documents often have multiple copies, both original and duplicate.  Ever buy a house?  It is normal to have to sign multiple 'originals' of the contract documents.  

What is the difference between having two originals, and simply saying 'I lost my original, can I buy another?' regardless of whether the original is lost.  

If you have a diploma, and you make a photocopy, frame it, and hang it in your office, and keep the original in a fire-safe, seems ok to me. 

For degree verification, I've been asked to provide official, sealed, stamped transcripts, not a copy of the diploma. 

In summary - the policies will vary based on laws, precedence, legal theory, and/or policy.