I personally think that halal food is marketing gone bat**** crazy. The principles behind it are nothing but honourable: only kill animals to eat them or use their skin, don't eat dead animals (as they could transmit diseases if dead for too long), have a prayer in respect for the animal killed, etc.
All this is very good but it is NOT compatible with industrial food. Have a recording of a prayer played to masses of cattle and have them 'technically not dead' is (to me) not in alignment with the intent of halal food.
Now the reasons why so many foods are getting certified is simply because there is a huge market for it. The costs of certification are not passed onto the customers because more products are sold and that outweighs the certification price.
All these theories of it funding terrorism is total non sense.