They act cold to protect themselves from more pain. If they didn't then they would fall apart and they cant do that. They have to keep going at all costs. Its how they survive the torment that they live everyday.
Yes, this is my understanding, too. It is the twin scourges of splitting and dissociating that allows pwBPD to turn off their emotions so quickly and so completely. It does indeed feel brutally cold. I know myself and I sympathize completely with how hard this is to deal with. And how much it hurts.
I'd like to say try not to take it personally, but I know very well how impossible that is to do. It's very personal and it hurts tremendously. In truth, I don't believe that pwBPD do these things out of malice. None-the-less, the behaviors themselves are abusive, cruel, and heartless.