This is what John Resig, creator of the Jquery framework, said about ExtJS / Sencha and their license policy:
According to the "show me your friends and I'll tell you who you are" wisdom, the Yii Framework project should only associate this intimately with projects that share its open source philosophy, not those that prostitute its spirit to peddle a license using subterfuge to make a buck. The ExtJS / Sencha company is using Yii's popularity and pristine reputation to lure users in, befuddle them with confusing license terms and get them to buy a license... it's very sneaky, disingenuous and makes Yii complicit in their despicable practices.
If you want to read the story of how ExtJS went from open source to corporate greed, including John Resig's quote above, read pablotron . org/?cid=1556