A Difference in Deference

Last thing from me about Kelo, of which I can only assume most of you are now thoroughly bored. One thing that troubled me as I dug through various Takings Clause cases was the repeated insistence that the Court must defer to legislative determinations of what constitutes a "public use" in all but the most outlandishly pretextual instances.

My question is: Why? They don't do that for a whole slew of other terms. The Court didn't feel compelled to defer to a legislative interpretation of "speech" that excludes conduct like burning an American flag. The Court doesn't just let states determine for themselves whether their laws provide "equal protection" or whether some investigative technique counts as a "search." If not in these cases, why do governments get to act as their own Webster's when it comes to "public use"?