>the US has a law that erases the tax bill for dead people

That is true, but there is a theory, applied very weakly, that supports this. The idea is that a decedent's estate is subject to a wealth tax on its fair market value, therefore to also subject the unrealized gains within the estate to income tax would be double taxation, which is to be avoided. The flaw is that the exemption from the estate tax is relatively high (something like $13,000K), so there would not be any double taxation is most cases, but it's treated that way nonetheless.