I agree with darky on this, it's basically for money and audience.
In the Uk we have 15 instead of PG-13, so here the audience was rather limited, but it was a concern for US audience only really.
Also if you look at resident evil(also Anderson involved), they were 15's in UK to gain more money, instead of depending on people over the age of 18, and they weren't too great either.
A similiar example to the avp situation is Rocky Balboa, compared to the old ones.