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.
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.
"When is license necessary:
1. Write applications on Java - No
2. Write classes that implement Java API - Yes
3. Download Java software components - Yes"
I think the stance from Google is that the answer to 2 is NO. And that 3 does not matter since they wrote their own Java software.