They didn't have to remove bacon to be able to sell 'halal' meat. Surely asda sells both in the same Isle and the muslims I've seen seem to buy anyway. ..
That's because the meat is sealed in a package and can't be contaminated. You'll find there isn't Halal meat on the fresh meat counter.
In Subway the person making your sandwich might touch Haram meat and then touch Halal meat without changing his gloves or equipment then contaminating it.