Legislating From The Desk

One of the more contentious issues in the confirmation hearings for Alito has been that of presidential signing statements.  

One of the core principles in neoconservative philosophy is the opposition to the practice they refer to as legislating from the bench.  

The principle of division of powers calls for Congress to write and enact laws, the executive branch to carry out the laws, and the courts to interpret laws.  Legislating from the bench is said to be the practice of a judge writing law, rather than interpreting it.  

Isn't it true, then, that the presidential signing statement amounts to the same thing?  When a presidential signing statement is issued, the executive branch writes an addendum to the law, with the intent of limiting it, nullifying it, or expanding it, or altering its meaning in some significant way.  

I'm trying to figure out how the same people could say that one of these is good, while the other is bad.