And while I certainly don't expect any of my referees to go over the code line-by-line, one of the rationales I had for GPLing the code was to make it possible for someone to replicate my study. One of the basic tenets about publishing research is that articles should contain all the information needed to reproduce the results. This is not possible for simulation papers unless the source code is public (or maybe a binary is provided, but that has many pitfalls by itself).
Results generated with proprietary codes could just as well be hokus-pokus in my mind. It's not clear how it adds to the field unless others can reproduce and build on the results. And with reasonably complicated codes, it's not realistic to think that someone will implement their own version (though that is of course the absolute best thing).
So while I don't necessarily think scientific software should be peer reviewed, I'm very hesitant about results obtained with proprietary software.