"Bob can incorporate Alice's code without making any changes to his license terms, while Alice would have to change her license terms in order to incorporate Bo

"Bob can incorporate Alice's code without making any changes to his license terms, while Alice would have to change her license terms in order to incorporate Bob's code" This is an inaccurate way of describing what happens. It buys into an incorrect idea of "GPL as virus". Alice's code remains Alice's, regardless of whether she incorporates Bob's code. Her licensing is unchanged. The combined work has to be distributed according to all licenses that make up that work. That includes the BSD and the GPL licenses. Because the BSD requirements are effectively a subset of the GPLs, and there is no incompatibility between the requirements (i.e., BSD is GPL-compatible), then it's as though everything was under the GPL. But Alice is free to deincorporate Bob's code, and everything is as it was.