IANAL, but I read that you have to sue to protect your property (copyright, brand name, etc). If you don't, you are waiving your rights on it, which is why Google may be using Classpath and Harmony as examples.

The requirement to defend ones IP generally only applies to trademark law, not copyrights or patents.

I don't think you're obligated to sue all infringers, and spend money chasing those those who don't harm your business and don't have the money to pay you should you win.