Semi-quoteth the source:
"<blah blah saving throw>. A defender who fails this saving throw is dazed for 1 round. Constructs, oozes, plants, undead, incoporeal creatures, and creatures immune to critical hits cannot be dazed."
While you can take the second sentence to mean exactly that, that if you're immune to criticals you cannot be dazed, I feel that the sentence implicitly means "by this feat". Otherwise, a character with heavy fortification could not be dazed period.
Like nauseated, dazed is a name for a game condition: that of taking no actions but not having a penalty to AC. The "dazed" state can be induced by many different things; an enchantment spell, having a heavy piece of metal slammed into your face, etc. It's a lot like swarm nauseating people; if you're in the middle of a bunch of centipedes you aren't actively heaving, but you're suffering a condition _that is called nauseated_.
They could have called Dazed "State 1", Nauseated "State 2", Stunned "State 3", and removed any possibility of confusing because of "real" definitions of the terms.