The VC firm that extends an exploding offer and won't negotiate it is guaranteeing that they'll only fund companies that aren't very good at business.
A few years ago I wrote:
It seems to me that a company that accepts an exploding offer is demonstrating a remarkable lack of basic business aptitude. Every building contractor in New York knows you request bids from five or ten plumbers before you award the contract. If a plumber said, "I'll do it for $x, but if you shop around, deal's off," the contractor would laugh his head off and throw the plumber out on the street. Nothing sends a stronger message that an offer is uncompetitive than refusing to expose it to competition. And that's for a $6000 kitchen installation. Getting $10 million in funding for a business is the biggest and most important deal in the life of a company.