This is where some BSD folks get frustrated by some GPL folks. Compiling code under BSD with code under GPL into the same binary is not _relicensing_. Each section of code still has the same license, and any binaries must abide by the intersection of the licenses. It is indeed the case that the intersection of GPL and BSD is equivalent to GPL and therefore allowed by the GPL, but the BSD license remains on the portions (of code and of binary) that are BSD licensed.
I read it as intersection of the permissions is a union of restrictions. So, I think the point is that given the union of the restrictions, you get the most restrictive license in this case, the GPL.
