Why rev canonical link always use 302 redirect

Some sites supports RevCanonical link for self hosting short URL. Such as flickr, php.net, dopplr.

For my understand and some articles also said that. URL shorten service should use 301 permanent redirect instead of 302.

But all the websites I lists above is using 302 temporarily redirect for their RevCanonical link.

Here are some result I got using curl -I

Dopplr:

&开发者_Go百科gt; curl -I http://dplr.it/explore/0xd1                              [0:57]
HTTP/1.1 302 Moved Temporarily
Server: nginx/0.5.33
Date: Fri, 04 Mar 2011 17:21:33 GMT
Content-Type: text/html
Content-Length: 161
Connection: keep-alive
Location: http://www.dopplr.com/dplrit/explore/0xd1

Flickr:

> curl -I http://flic.kr/p/9m9Cmj                                  [1:21]
HTTP/1.1 302 Found
Date: Fri, 04 Mar 2011 17:29:43 GMT
Server: Apache
P3P: policyref="http://p3p.yahoo.com/w3c/p3p.xml", CP="CAO DSP COR CUR ADM DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE GOV"
Location: http://www.flickr.com/photo.gne?short=9m9Cmj
....

I don't understand why this case(self host shorten URL) should use 302 redirect. Can anyone explain the reason?

As Franz also notes above, in this particular case, 302 is not a good decision.

Here is Matt Cutts discussing your exact question:

Q: Why are you doing this?
A: Google needed a url shortener for its own products where we knew the shortener wouldn’t go away. We also wanted a shortener that we knew would do things the right way (e.g. 301/permanent redirects), and that would be fast, stable, and secure.

http://www.mattcutts.com/blog/goo-gl-url-shortener/