Why does it go out the door? Because you say so?
You're entitled to that opinion. Or, you could see Rand's move as disappointing but "tactical."
Sometimes, you opt to play the game. It's fair of you to criticize Rand for this, but you ought to consider this basic fact: everyone compromises in some way. Ron Paul chose to work within the Republican Party because he knew it was the only realistic way to get elected and have influence on the agenda. Ron Paul could've protested the immoral and unconstitutional tax laws of this country, but he didn't (see his interviews where he salutes people who do so out of "civil disobedience").
We are all faced with these dilemmas. Do I comply for the sake of expediency/my family/my welfare? Do I go along in order to influence the agenda from the inside? Or do I resist, and if so, how?
Again, I disagree with Rand signing this letter. And I get those who are more outspoken in their disagreement. We all must decide for ourselves how to answer the questions in the above paragraph, and we're all free to evaluate our elected officials' decisions in that regard.
But we have to put context into the equation when doing so. Saying everything "goes out the door" over something like this is setting a standard that, in my opinion, almost no one could live up to--and no one who managed to live up to it would be able to gain influence to effect change.