In heraldry aren't they looking at the flag as a design, rather than the actual object on a flagpole? By that I mean during the designing of the flag one starts with a field of colour and then adds details onto that field, like the layers of a painting, one on top of the other. But when one puts the flag up a pole, the flag is everything, the material, the field and the specific design, it's all one package. A fly lands on this complete package call the flag, or a bird puts it's mark on the flag, but the flag itself only contains the original design, not the fly or bird droppings.
In English heraldry one typically starts describing a blazon with "on a field…" yes. National flags, at least in the European nations, started out as blazons.
But, as was said, this could be a quirk of English. The few book(s) I've been able to find on Heraldry in Esperanto (including this web page: http://www.bertin.biz/index.php?titre=priskriboDeBlazonoj.htm) suggest that the "On a field…" bit is superfluous and just start with the base color(s) and then "with" other objects, shapes, etc.
I've got an SCA† story about a new fighter with a blank shield that got a bird dropping on it. SCA Heralds are pretty much all noted for their attempts at humor. ("We don't pun, we Cant¶" being one example.) So his shield became On a field argent (white) a gules (circular shape), melting, purpure* (purple)
† Society for Creative Anachronism.
¶ Cant being an olde word meaning "word play"