They aren't. When you seq over a map you get a list of MapEntry, like so: (clojure.lang.MapEntry. :a 1)
MapEntries aren't vectors, and while they can be destructured like vectors, you can't make empty instances of them.
(empty) still is useful in a variety of cases. See for example this bug report that I've filled: http://dev.clojure.org/jira/browse/CLJ-1476
user=> (pr-str (seq {:a 1 :b 2}))
"([:a 1] [:b 2])"
user=> (vector? (first (seq {:a 1 :b 2})))
true
user=> (doc vector?)
-------------------------
clojure.core/vector?
([x])
Return true if x implements IPersistentVector
nil
empty mapentry returning nil is meh. If it returned an empty vector it'd be what you want 99% of the time.