It doesn't; the Show instance for Data.Map, however, does a toAscList and then wraps the resulting list in a fromList, because the internals of a Map are deliberately hidden.

If the internals of Map were visible, it would be possible to build an invalid Map; at best it would be unbalanced and slow, at worst it would be misordered and keys would randomly go "missing" as a result.  See, for example, how (Map Double a) misbehaves (try searching the haskell-cafe list for "Map Double"):  the poorly defined Ord constraint on floating point types, especially in the presence of NaN, means keys *do* sometimes go missing in this case and (Map Double a) ends up being rather unreliable. 

This also means you can break a Map by providing an incorrect or inconsistent Ord instance for its key.  This is why typeclass instances are global; if there were local instances, you could do severe violence to a Map by providing an incompatible/incoherent local Ord instance for the type of its key.

A node in a Map is either a Tip (placeholder of sorts) or a Bin comprising a strict number of subnodes/values, a strict key and its associated value, and strict left and right subtrees (which may be Tip-s if this is a leaf).  You should never try to work with this directly (and under normal circumstances you can't, as the constructors are hidden), as outlined above.