The short answer is that Dryad Arbor can't be countered because it can only be played as a land, not a spell. Here's the supporting statement from the card's Oracle rulings:
And the card's reminder text:
If you prefer rules over rulings, let's start with the rule saying you can't counter land:
Moreover, you'll want to note this section of the comp rules:
Essentially, this rule is saying that the Land card type overrides the normal behavior of the various spell card types.
This rather esoteric rule isn't actually required for Dryad Arbor to work the way that it does -- simply having no listed mana cost would mean that you can't play it like a normal spell, while being of the Land type would allow it to be played as a land. My best guess as to why it's there is that the original multi-typed lands, Mirrodin's artifact lands, were much more "land" than "spell," so this was codified in the rules for playing them. I take it as not so much saying that a land with a mana cost would still be uncastable as precluding the existence of such a card altogether.