Thanks for your responses. I have to do some research on signing statements as a general matter, but here's my take so far:
Isn't a signing statement nothing more than a written record of how a president intends to enforce the law he is signing? If that's correct, then the idea that a signing statement "carries the force of law" kind of misses the mark. No court would be bound to judge a case based on a signing statement. No later president would be bound to a former president's signing statement (they are not an executive order, more a diary of intent, but even if they were exec orders the next Pres could override with his own order). Signing statements don't alter the law being signed, their only power is in the executive branch's power to enforce the law. They "carry the force of law" only for as long as a president adheres to them in execution of his office and enforcement of law.
So the question really is this: Is a President correct to not enforce a provision of law that he believes is unconstitutional?
Certainly a President gets to weigh in on the constitutionality of bills by either vetoing or signing them, but once they are law he has a duty to enforce them. I know, this may sound counter intuitive. If a law is unconstitutional then it obviously should not be enforced, right? I say wrong.
There are only two ways to get a suspected bad law off the books. Repeal it through new legislation or get it overturned in a court. If a legislature just passed a law with unconstitutional provisions, it is safe to say they aren't about to repeal the law unless there is some blatantly obvious oversight involved. That only leaves judicial review as a practical way to get a suspected bad law off the books in relatively short order.
If a president doesn't enforce a law believed to be unconstitutional then how can anyone try to get the law overturned in a court? In order to do so the law must be applied in at least a token way so that there is someone with standing to bring a case before a court. If a president doesn't do so, even if it is against his better judgment, then the law will stay on the books, a time bomb waiting to go off when the next President comes along. That president may make a non-token enforcement effort, one with far greater detriment to society than would have happened under a token enforcement effort.
And the President is only one man and is therefore fallible. Making the token enforcement effort to invite judicial review is a humble act and a hedge against a President's bad legal and perhaps moral/ethical judgment. The people and the President him/herself are protected against bad law by a token enforcement effort and allowing the challenge to come through. If he's right and it is found to be unconstitutional, then a great service has been done by getting the law off the books. If he is wrong and the law is found to be constitutional through judicial review, then obviously it is incumbent upon the president to enforce it to the extent possible.
The only class of provision in a new law that would be tricky is when the legislature tries to force the hand of the executive in carrying out actions that are traditionally the sole purview of the executive. An example might be a provision where the President is mandated to adjust the deployment levels of ICE agents in a region. The legislature can certainly cut or increase budgets, but trying to insinuate themselves into the command structure and the tactical/strategic deployment of law enforcement (or military) is another matter. In that kind of scenario, there is no token effort that can be applied and there is no legal challenge that can reasonably come from a criminal or civil case. One either adheres to the provision or not. In those cases, it is a purely political matter and signing statements are as good as anything to get the message across to the legislature (and the voters). And as such, the only options are to let it slide, to pass a budget change to press the point, to impeach the president, or to trumpet the act (or rather non-act) as a campaign issue when reelection season comes around.
Hopefully those kinds of provisions are far and few between, if they aren't then the legislature needs tighten up its game. But the President too needs to be very careful not to try to fit every provision to this type of case. Bush quite likely got carried away with the use of signing statements. For example, striking out 2000+ provisions in a single bill sounds excessive. Of course many bills are excessively large and stuffed full of crap that doesn't rightly belong there in a sane world. I also believe that congress is full of a bunch of political chickens and quite likely got carried away themselves in trying to make Bush do what they wanted without using the politically tricky, valid tools they had at their discretion, namely control of the appropriations process, budgets, and control of the legislative agenda. The fact that even Obama is using them, particularly this early in his tenure with the most politically pliant and aligned congress a President has seen in many many years... I tend to think congress is more responsible for these signing statements being necessary than the President is for having to use them.