Why does flatbread have to be microwaved / cooked / toasted?

Most likely the flatbread is not very pliable when cold. I believe that Subway forces the heating of the flatbread to keep it from splitting when they fold it.

Consider the humble supermarket pita. The kind with the ridiculously long shelf life.

Straight from the packet, it's pretty miserable. It's chewy - not in a good way. It tastes of cardboard.

Toast it however, and it comes to life. The slight browning of the outside improves the flavour. The steaming of the inside softens the bread inside.

I'll bet the Subway flatbread has similar properties.

Now, if I ran a fast food chain that sold sandwiches based on a bread like this, I'd make sure my staff were trained to always toast it. Even if a customer asks for it untoasted, that would mean they'd go away with a sandwich that I know won't be very nice. I would rather take away the customer's choice, than sell them something that's not good. Subway does not want thousands of customers telling their friends the flatbread isn't nice.

If we're talking about the same sub making restaurant, their flatbread contains the following ingredients:

Enriched wheat flour (flour, niacin, reduced iron, thiamine mononitrate, riboflavin, folic acid, malted barley flour, ascorbic acid), water, soybean oil, yeast; contains 2% or less of: nonfat dry milk, wheat gluten, salt, dough conditioners (guar gum, Arabic gum, sodium stearoyl lactylate, enzymes), sugar, baking powder (sodium acid pyrophosphate, sodium bicarbonate, corn starch, monocalcium phosphate), calcium propionate.

I can see nothing in there that isn't in found in commercial soft white bread. It may well be a case of an overzealous legal department acting on the merest possibility of a hint of an idea that one ingredient may perhaps have the potential to maybe sometimes cause a problem if not cooked.

The reason subway has to "heat" the flatbread is to make it soft. If you want it toasted you get it with the meat and cheese toasted under a hotter setting that actually toasts it. When subway released the flatbread they understood some people don't want there sub toasted so in order to comply with this request they have the option of just heating the flatbread without the meat and cheese on it under a heat setting in the toaster. It doesn't toast it just warms it up enough so it's soft and doesn't break apart or crumble when they fold it or while you eat it. I know most people aren't use to being told they "have" to order something a certain way but believe me if you don't heat the flatbread it's not edible. Unless you like dry crusty crumbly bread with stale taste.

Having just eaten an untoasted Subway flatbread sandwich, I noticed the bread tasted like flour and had an unpleasant texture. So they "have" to toast it to make it taste good, not for any weird reason about chemicals.

Some restaurants may prefer to toast fladbreads slightly in order to serve hot, but unfortunately the chef in question here seems to be either not too good at explaining things in English, or may have just tried to convince the customer that the toasting requirement was something serious.