Zi Yang,

ALL those premiun/pro/paid/etc... WordPress Themes & Plugins absorb the GPL license. Hence technically speaking I can go to ThemeForest (or whatever your choice of theme/plugin source is) pay for them and redistribute.

Every theme/plugin knows about GPL before they start their themes/plugins.

Those authors might think this is not cool but...oh well.

Not saying I would do this. I think it's stupid to go to TF, pay for things, then distribute things for free...money losing proposal.