“Matt 15:11 Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man.”
The New Testament prohibition against eating meat from idols has to do with not financially supporting false religion or giving the appearance of condoning the false religion.
Your point -— that the food itself is harmless, yes. Prayers and ritual sacrifice to allah are just meaningless mumbojumbo and have no effect on the food -— stands and is correct.
But it is a different issue.
Yes, I understand the danger of appearing indifferent. But I don’t think eating at Subway gives out the message that one is condoning Islam.