It boils down to chemistry. The nickle-metal-hydride chemistry used in rechargeable batteries just don’t produce 1.5V (explaining why is a lot more complicated), while the alkaline chemistry in non-rechargable batteries produces slightly more volts.In the same vein, lithium-ion batteries produce around 3.6V, while lead-acid produce about 12.4V.

To answer your other question, they should be able to power most things that aren’t an analog clock (which might run slightly slow).

Even then, anyone can use inspect element to make it look like it says whatever you want.