Answer

This is not my invention. It is the position of all the halakhic decisors, and it is explicit in the Talmud (although in Maimonides there is a contradiction on this issue, and it depends on different rationales found in the Talmud itself: the presumption that she checks carefully before marrying, or the enactment for abandoned wives). The explanation is very simple. If, as a matter of fact, the husband died, the woman is permitted to remarry. True, there is a rule in the laws of evidence that only two witnesses can permit something involving forbidden sexual relations, but that is only a rule of evidence. Therefore, if the Sages are convinced that the husband really died based on one witness (and on the presumption that she checks carefully before marrying), they can be lenient in the laws of evidence and permit her to remarry. They did not permit a married woman to the world at large; they only relaxed the laws of evidence.
In principle, the Sages can uproot a Torah law through passive omission, and when there is great need then apparently even through positive action. If the permission is to marry, then that is permission through positive action. But if the permission is not to require testimony from two witnesses, then there is room to discuss whether this is passive omission or positive action.