Reneging on an offer means revoking it after it has been accepted, and that's poor form.
Turning down an offer puts you into a small category of "people we would hire if we had the chance" and the recruiters or hiring managers will follow up with you for some set period of time just in case something changes on your side. They already have decided you would be a good fit, after all.
But here's what I don't understand about this: wouldn't that be the company revoking the offer, not the employee? If the company revokes the offer or has "exploding offers" or whatever, that's a corporate thing, not an employee issue.
2. Company B says "we'd like to hire you at $X/year"
3. Person A says "that sounds good, I'll start in a month." Company B stops trying to fill the job.
4. Up to a month passes.
5. Person A tells company B "lol nevermind" and doesn't actually start work. Normally this is because company C is offering to pay $Y and $Y >> $X.
It's an employee issue because the employee is not following through on what they said they'd do. Because employment is at-will in the US they're legally in the clear, but the company that was planning on hiring them is still kind of screwed.
