>all portable electric space heaters generally put out the same 1,500w
It really doesn't matter what they put out, all resistive heaters are 100% efficient (any inefficiency would result in heat anyway).
Are people buying new miners? Where does new Bitcoin come from - already amortized ones? Still, if that's what you have, it's better the heat from them than from a resistive element.
I don't see where marketing heat from a miner is any worse than the "Amish space heaters" which sell for $500 and make invalid claims about having better efficiency than a $40 milk house heater.