I agree with you. For me a traditional Scandinavian earth burial - going back to pagan times and Indo-European traditions - would do just fine. But it is important that we also understand that this is not sufficient for many traditional Zoroastrians for whom earth, water and fire may not touch the corpse, this is how sacred they regard the earth and how low they hold a dead body (as its opposite to a human being and not a human entity on its own). In that case, earth burials will not do, so recycling has become the favored option (as it is among Tibetan Lamaists too, who also feed corpses to vultures). However, understanding and sympathizing with a tradition does not mean we have to follow it. Zarathushtra for one did not care. So for you and me as western converts to Zoroastrianism, we are likely to construct our own habits that go with our faith and we should all respect each other for these various choices we make.
I guess it all depends on what you perceive as defiling. I can understand if you have a thousand years of religious teachers believing that corpses defile the land, how one might be adverse to burying the dead. But speaking for myself, I would rather the worms and bugs have me and then I enrich the ground right then and there, rather than the vultures having me, then poop me out from hundreds of feet in the air to fall upon...whatever. I know I wince at being driven around on the roof of a car in, say, Nevada at 115F heat in the summer for three months until the next rain, or until someone feels like washing me down a car wash drain!
However I would agree that modern methods of burial do defile the land, with all their embalming fluids and shellacked caskets and polyester/rayon etc. casket linings, and suits of clothes and other trash that we put in the ground to make ourselves feel better. Just wrap me in soft cotton and lower me into a hole. The good Earth will make me useful to her creatures in a speedy and dignified way.
I do not agree with the ancient teachers and their descendants, that one can defile the fire or the earth or what have you with burials, nail clippings or stray hairs. Indeed, I understand the earth, fire and water to be agents of purification, and that discerning Asha teaches us how each one works to make clean that which might have been unpleasant and yucky, or just plain unsafe, before. For instance, if there is no rubbing alcohol, put your needle or knife or what have you into the fire to burn off bacteria and viruses before opening sores or whatever. The only way I can see to pollute a fire is to dump harsh chemicals into it that will make the air and the ground around unsafe for living beings.