mtb/bmx manufacturers put cheap plastic pedals on full build bikes because pedals are a personal preference and the majority of the time, the rider will have a pair of pedals they'll want to put on it. These plastic pedals are for riding the bike home after being bought.

 Then how do you explain the massive selection of aftermarket plastic pedals in the bmx section of most shops

 you've missed the point a bit, the pedals pictured are shitty stock pedals, they're not gonna last. The plastic aftermarket pedals you can buy (gussets etc) are actually pretty decent, light, fairly strong and grippy enough for dirt/park/street as they're actually designed for that specific use. I know a few people that ran them and they didn't break.

 I understand the cheap pedals that come on a bike. I still don't get the plastic after market ones, I have seen a huge selection of broken plastic pedal bodies on here.

 plastic pedals will not hurt that much if you get a pedal bite, are better for grinding cos they slide much better, same whit plastic pegs, on grinding they make less noise, are light and much cheaper to make. those are the good sides of them.