This (planting letters to the editor) is just one of the sleazy tactics which PR firms use to promote the products of their corporate masters. Other tactics include everything from producing bogus "Video News Releases" (which are often picked up by local TV and radio stations and broadcast as "news"), to infiltration and subversion of activist groups. For more on how the PR industry works, the book Toxic Sludge is Good for You! is a must read. Also check out PRWatch.org.