This is only partially true. You are 'strongly encouraged' to enforce your patent violations, for after a period of years (6) they basically become unenforceable. This is to prevent things like 'patent bombs'.

In practice it doesn't always work this way ( see patent trolls ) but it can be a BIG issue for people looking to enforce their patents.

Here's an interesting example:

http://www.ipwatchdog.com/2010/05/26/estoppel-time-bomb/id=1...