Just to add my own opinion in to the mix.
I've spent the last 9 years designing and installing factory management systems (both hardware and software) for the food industry, so it's fair to say I've seen more than my fair share of meat factories, abattoirs, and processing plants as well as the occasional Chicken and Cattle Farm. I've worked closely with a lot of Halal processing plants and abattoirs, and I'll say a few things:
1. Slaughtering of any animal is not a pretty thing. Step in to any slaughter line and you'll want to have had a light lunch and strong stomach.
2. I strongly oppose the unstunned slaughter of any animal, and given the choice I wouldn't eat Halal meat at all (even though I have it on good authority (from a guy who runs a very large Halal slaughter house) that the Halal method gives more tender chicken as it drains more of the blood from the meat). - Personally I think that if something is 'unstunned' it should be labelled as such, though given the small amount of factories practising this method you'll probably never find this meat in mainstream outlets such as Asda, KFC, Subway, McDonalds etc. anyway, but I would rather that I could see this information and decide not to eat it.
3. If you really care for the welfare of any animal, always buy free range. Always.
Coming back to Subways decision to stop selling Pork products. I disagree with the decision. As an atheist I disagree with any decision that restricts peoples choices based on others religion (be that Christianity, Judaism, Islam, or Jedi).
HOWEVER; Subway are a BUSINESS. They didn't take this decision for Religious reasons (can you really see an American company publicly wanting to be classed as 'Muslim'?). They have made the decision to firstly start selling Halal, then to remove Pork as it increases profits. If you want them to stop you have to do exactly the same as the religious groups would have done if they didn't make these decisions.
1. Publicly express your issues in a way that subway take notice (post on facebook, twitter, Instagram etc) with the #subway - They have departments set up to monitor these things, but best to keep race out of it!
2. Stop buying from Subway. If they can relate a drop in revenue with the recent bad press around their decisions they'll soon fetch back the bacon we all want so much. Or, simply do what a what a muslim would, ask is the meat Halal, or if they server Pork. If they only serve Halal or don't serve Pork and you don't agree with it then don't buy anything and leave.