What prevents the following situation?
- Alice writes Alice's Great Open-Source Program and releases it under the GPL.
- Bob creates the derivative work Bob's Great Open-Source Program With Racing Stripes, which is just Alice's program with racing stripes on it, and dual-licenses it under the GPL and another license.
- Charlie creates Charlie's Closed-Source Program With Racing Stripes based on Bob's program and releases it under a closed-source license only.
I don't see anything in the GPL v2 preventing this. Particularly, I see a requirement that Bob release his program under the GPL, but I don't see a requirement that he not also release it under any other licenses. (Now that I've checked the GPL v3, I can see that version has a sentence saying "This License gives no permission to license the [derivative] work in any other way". Not sure how much legal effect that has, but it's there.)
(Does it matter if Bob's modifications are more substantial than just racing stripes?)