Cool video. Doing the double-slit experiment in my freshman physics class is a favorite memory from college. Seeing it in person blew everyone's minds, even the kids who had learned about the experiment before.
If you google "is energy conserved in the double-slit experiment" you'll find some physics forums with decent answers. Basically, the total energy emitted by the light source does not change. Energy is conserved. Don't think of the laser light as a discrete beam that is being split off onto a second path. Instead, imagine that the laser light is constantly shining all over that foil and card. The dark regions appear dark because the light waves there are canceled out by interference from adjacent light waves. Similarly, the red areas are illuminated because in those areas the adjacent waves did not cancel each other out. The bright spots visible on the polarized foil occur because the polarizer blocks thin regions of the light, preventing them from canceling out adjacent light that wasn't blocked. So light wasn't redirected there, but was always there and was simply made visible to us by the effect of the polarizer.
Light, quantum mechanics, and the probabilistic nature of the universe are all real head trips. I still struggle to wrap my mind around them. As such, there's a good chance my simplistic paragraph above is incorrect or misleading, so take my answer with a grain of salt.
At this point I have to admit that I am out of my depth. We need a real physics major to chime in.