The Halal thing is just business, not Islam taking over.
If you have two groups of people:
a) People that don't care where their meat comes from or how it is slaughtered (90% of whiteys)
b) People that can only, according to their religion, eat Halal meat
Then you may as well slaughter it to please option B. Because that effectively pleases everyone.
My two cents... let's be honest we don't really know what goes on in slaughterhouses, and everything that I've ever had brought to my attention through the media is totally horrendous and I just ignore it because it makes me feel bad. I'm not alone in this. Halal or not, an abbatoir is not a nice place, animals are bred for food, if we spend much time thinking about it we'll all go veggie, so I do what nearly everyone else does and just pretend that it doesn't exist and bacon comes from bacon trees.
I doubt Halal slaughter is much worse than a lot of the treatment that goes on in slaughterhouses and if they're all stunned beforehand then who cares, honestly.