Here's the legal problem:
Did both you and the customer have a "meeting of the minds" about what was being purchased?
Did you specifically state what was not included in the purchase?
I'm gonna guess the answer is "no" to both questions.
Years ago, printers printed a list of "trade customs" on the backs of their invoice and (sometimes) estimate forms.
That practice was abandoned about thirty years ago because courts said it was restraint of trade and pretty much unenforceable. (After all, if ABZ Printers
wants to give away negatives, why should they be restrained from doing so by "trade custom"? And, "what's with all this tiny type on the back of an invoice when I wasn't advised when I placed the order?")
Unfortunately, most printers did not bother to come up with a replacement way to tell what the transaction involved. The printer/buyer transaction now has very muddy responsibilities and rights.
In short: contractual responsibilities, if not defined by a written contract, can be defined only by a court. Which means several things.
First, a prudent printer should have a standard "we're gonna do business and here is how we do it" contract that is signed before starting any business relationship.
(Ah! but having to sign a contract could dampen the enthusiasm of many prospective customers! Indeed. And that is a crucial factor to keep in mind.)
Second, at every stage of the production process there should be a reminder of the rights you claim.
Now, both of these principles are observed only in the breach by most printers. For the sake of expediency or something else.
What would be a reasonable expectation for a purchase from another craft, for example photographers?
I state photographers because they are usually MUCH smarter than printers about this stuff. You are ALWAYS asked to sign waivers and releases and stuff by any photographer who's been in business for more than half a year. For good reason.
There is no simple solution. You can drive away customers either by pre-defining your stand or by making your stand at the moment of conflict.
What are your rock bottom "rights"? Very few, if you don't take the care to define them before you accept the order.
(And don't think that a court would "just naturally" take your side. There are MANY cases over the past thirty years that have been judged against printers following every single piece of advice that has been given here about releasing "bad" art, art without linked files, etc., etc. Caveat venditor.)
What should you do? Be pragmatic, whatever that means.