There's a major bug in the current Combat implementation: Prey on the Weak damage bonus is only applied on special attacks and not the base auto_attacks.
To validate the bug, I modified the 18_51_02_FD profile to use this (pretty stupid) spec:
Talent Calculator - World of Warcraft
Here are the melee results for the base spec and the modified one:
base (with PoW):
melee_main_hand Count=256.2| 1.2sec DPE= 1645|22% DPET= 1409 DPR= inf Miss=12.8% Hit=1250 Crit= 2584| 3748|47.6% Glance= 940|24.1% Dodge=0.4%
melee_off_hand Count=476.8| 0.6sec DPE= 651|16% DPET= 1038 DPR= inf Miss=12.8% Hit= 495 Crit= 1023| 1487|47.6% Glance= 372|24.0% Dodge=0.3%
sinister_strike Count=115.3| 2.6sec DPE= 3357|21% DPET= 2919 DPR= 88.3 Miss=0.0% Hit=1760 Crit= 5137| 6180|47.5% Dodge=0.3%
modified (without PoW):
melee_main_hand Count=256.1| 1.2sec DPE= 1641|24% DPET= 1405 DPR= inf Miss=12.9% Hit=1250 Crit= 2582| 3748|47.4% Glance= 940|24.1% Dodge=0.3%
melee_off_hand Count=476.7| 0.6sec DPE= 649|17% DPET= 1034 DPR= inf Miss=12.9% Hit= 494 Crit= 1022| 1487|47.4% Glance= 372|24.0% Dodge=0.3%
sinister_strike Count=115.1| 2.6sec DPE= 2906|19% DPET= 2527 DPR= 76.5 Miss=0.0% Hit=1759 Crit= 4188| 5044|47.4% Dodge=0.3%
It's quite easy to see that sinister strike damage is correctly affected but not the base melee damage.