How can a company sell licenses for GPLed software without violating rights of others

There are tons of questions like "Can I use GPLed code X in my commercial project?" but I found nothing which answers my question.

I take ExtJS as an example, but actually you can any commercial / community edition dual licensed software.

If I contribute to the community edition, isn't my contribution GPLed?

Even if my contribution was small (like a bug fix) how come they're allowed to sell it?

Disclaimer: I know, we're not lawyers. Just wondering.