A fairly detailed (if opinionated) answer to your question is here:

http://www.zephoria.org/thoughts/archives/2008/02/06/openacc...

The summary is: (1) academics have historically given away their research and reviewing work for free to (revenue seeking) journals and professional associations to get distribution and prestige, (2) this no longer makes sense with the Internet, and (3) academia, journals, and professional associations are in an awkward transition as a result.

In the meantime, just use Google Scholar and hope that you are reading something important enough that someone has possibly violated copyright and posted a version online without a paywall.