Obviously the reason they put in level 2 pokemon was to give you easy training at the beginning... oh god, how I cringed when they changed the level 3 poocheyana (which, stretched to to the extreme, might be vaguely scary) to a level 2 zigzagoon...uh... so lame... I guess it was for all those trainers who had trouble beating the poocheyana, despite the fact that it runs away when your pokemon gets too weak [blatant sarcasm]
I dunno the justification, though... do all pokemon really lay eggs in the wild? It's just that that seems very unlikely. I always thought that the level 2, 3, etc were pokemon that were just born the regular way... wait, then where are the adults? Ok, I dunno then... just a very weak variety of the pokemon that lives around that town, but it was simple to classify it as the same kind at a low level?