Companies share each other's patents through patent pools. Such patent groupings protect companies from infringing upon patent rights from inventors outside the patent pool.
Best Patent Pool Article For Newbies:
Good article on patent pool organizations. There are too few patent pool organizations. Most technology pools focus on specific fields of technology: http://www.ipwatchdog.com/2012/12/10/defensive-patent-pools-there-are-surprisingly-few-options-2/id=31233/
• Micro Patent Pools: Example from Unified Patents http://unifiedpatents.com/solution.html
Patent Pools & Setting Of Industry Standards:
Standards Patent Pools for cross licensing