I think it's really unfortunate that postdoc sites are in a position to push us into these kinds of decisions, and in my view, it would serve them right if an applicant who was pressured into taking an offer later changed his/her mind. Especially when sites make absurdly early offers - it seems like they're really taking advantage of our anxiety about not getting another offer.
However, I also think that it's potentially a huge bridge burner, and I'd be really cautious about going back on an acceptance, even a verbal one.
I get that "real jobs" are like this as well, but "real jobs" also pay a more livable wage and are more stable (e.g. last longer than 1-2 years). They're also on less of an academic time frame. If you need to fill a staff position after someone leaves unexpectedly, I get that you can't wait around while an applicant decides whether to accept your offer. I don't get how postdocs justify needing a decision in 2 days.