It is a taken portal but I dont believe we are taken. Since the vanguard sponsore this game, the pvp aspect is probably more like crucible. The better question is why the adds see invaders as friendly? The only answer that really makes sense is this is some sort of jury rigged vex simulation that the drifter created. This would explain why enemies spawn in specific areas and are friendly to invaders. This makes even more sense since the splicer activity is basically gambit, but is a vex simulation.