I've a question about search engine copyright issue, 
I know the news and factor can be quota as "fair use"
ike google search engine, it give all the page a "cache", 
it's should be carefully consider all the legal matter,
but I just want to know, what is the relevant clauses
in the law allow the search engine "cache", are there a
time limitation for the cache? or it can cache forever?
for the google news, why they donot provide cache function?

I cannot speak on behalf of Google or provide legal advice.  (See the
disclaimer at the bottom of this page.)  My impression is that there
is no definitive answer to this question.  On the one hand, some
people have posted comments on the Web stating their opinion that
caching of web pages (on Google or the Internet Archive Wayback
Machine) is a copyright problem.  On the other hand, some people have
stated opinions that it is not a problem; moreover, it does not seem
that any court has ruled that it is (or is not) a problem.

A case that comes up in many of these comments is Kelly v. Arriba Soft
Corp., which involved copying of images.  Commenters on both sides
have used this case to support their opinions.

"Kelly v. Arriba Soft Corporation" (Feb. 6, 2002)
U.S. Court of Appeals for the Ninth Circuit
http://www.ca9.uscourts.gov/ca9/newopinions.nsf/C38AD9E9A70DB15188256B5700813AD7/$file/0055521.pdf

You might find it interesting to read this decision, and to search on
Google or other search engines for the words kelly, arriba [or ditto,
the name of the web site], and variations of the word "cache", in
order to read comments relating to this case.

Google and the Wayback machine (http://www.archive.org) will gladly
exclude a certain website or cache if a standard robots.txt file is
created on the website to tell robots to stay away from listing,
archiving or caching content. Given that it is so easy to set up this
file (which will not only prevent content from being on Google, but on
other services such as Yahoo etc.), I don't see why anyone would
bother to sue Google when they can fix it in a matter of minutes.

For an example of a robots.txt file see:
://www.google.com/robots.txt

Google news does not cache probably because the content changes so
often, and it is the preference of news companies not to have their
items cached.

For more information about robots.txt see:
http://www.searchengineworld.com/robots/robots_tutorial.htm

Thank justaskscott-ga & tisme-ga's comments, What I really 
concern is the exactly clauses in the law.  I know robots can avoided
it, it just like if you lock you bicycle, it would not be taken by others
but the problem is if you forget to lock you bicycle, then sb,take you 
bicycle a few hours, is it consider being stolen, or fair use?