It's deliberate, according to them. If you see people removing you from their friend list, it may be upsetting. And people probably do it a lot just to clean their friends list, either to keep it to a small number they can manage, or to keep within the 200 limit.

I think however that if you actually add someone to your blacklist that it should remove them on both ends, because at that point there is obviously something really bad going on.