Negative resistance in a regenerative radio receiver?
Why is an amplifier with a tuned circuit and positive feedback, such
as a regenerative radio receiver, said to exhibit or have negative
resistance?

It's a term I don't particularly care for that much. In the context of the question, it means that the losses of a tuned circuit (represented by real resistance) are reduced by the use of a little positive feedback. That reduction in real resistance is said to be adding negative resistance i.e. it cancels out the real resistance of a tuned circuit a little. Thus, Q factor is increased making more selective tuning possible.
It's not particularly miraculous or a specialty of the regenerative receiver - we do it all the time with active filters; it's just that the hype merchants have jumped on the bandwagon and introduced the term &quot;negative resistance&quot; as the means of cancelling regular (lossy) positive resistance so that Q factors of tuned circuits can be increased.
After all, do we make a big deal out of the Sallen-Key filter (that uses resistors and capacitors and an op-amp) for creating an invisible inductor by the judicious use of the op-amp and how it's connected?
Do we make a big deal out of a Colpitts oscillator and say that negative resistance is introduced? Some folk do but, in reality, it's just that the poles of the tuning components are brought to a point where they coincide with the \$j\omega\$ axis. That moving of the poles is because we reduce the tuning component's losses to zero. Call it negative resistance if you will but it hides the truth a little in my opinion.
Do we use the term negative capacitance when referring to an inductor. No we don't but, it's exactly the same principle; negative capacitive reactance is numerically the same as a value of normal inductive reactance at a given frequency.