Is an automatic copyright transfer clause in publisher's instructions to authors typically enforced/enforceable?
A lawyer would be best suited to answer this, but as I have had several business law classes as part of my business education, I'll hold forth based on my understanding of intellectual property and contract law in an educational context. 

The answer would be yes, that it should be generally enforceable. 

Laws vary by state even in the United States, but in general, qualities that can make a contract unenforceable are:


Lack of capacity of the parties (legal age, sound minds, etc.)
Fraud or misrepresentation
Violation of public policy


None of the above apply to our situation (unless, for example, we have a precocious or senile scholar).

Enforceable contracts also require:


Consideration (exchange of something of value)
Meeting of the Minds (mutual agreement of terms)
Offer and Acceptance


Now let's analyze these requirements. Is there mutual exchange of value? The journal appears to offer that in exchange for copyright, the author gets to be published in a prestigious journal. Does the author agree at the time of offer and acceptance? Then we have an enforceable contract.

So this brings to light a strategy for attempting to retain copyrights of your work. Insist by striking through such language in a contract and make a statement that you, the author, retain copyright, and put your initials by the alteration. If the contract is in electronic form, communicate this through email just prior to and just after submitting your request electronically.

If they do not object, and rather, accept your submission, I would assert that you have established that any possible claim by them to your work is either completely unenforceable (certainly, in the case of a written contract) or (in either case) liable to cost them a great deal in legal fees to pursue.