The distinction is this: "exploding" typically refers to a very short period of time, designed to prevent shopping the deal, extensive legal review or advice, or exploring other options. A legitimate deadline for an offer wouldn't be called exploding if it spans many weeks and lets a participant fully understand it.

Clearly, no offer (in any realm) will ever be perpetually valid. Giving a reasonable amount of time to evaluate options and make a better decision is considerably less explosive than one that puts an artificially tight deadline on the decision.

I would think it odd for any kind of business deal to be extended and awaiting a response without any kind of deadline at all.

I think exploding in this sense is more "this offer comes off the table at midnight" kind of stuff.