The voltage of a battery is dependent on what materials it’s made of and their electrical properties. So because of quantum physics and shit, a cell made of different materials will have a different voltage. The only real ways to change the voltage is to put multiple batteries in series like how there’s six little batteries in a nine-volt battery (but two NiMH cells would give 2.4 volts and overvolting can cause even more problems than undervolting), or have some kind of active circuitry to adjust the voltage (which adds a significant amount of inefficiency and therefore reduces the effective capacity of your battery, not to mention the cost of having a circuit board with many components embedded in the battery).

It really came as a surprise, is there a catch? Are they only good for low power stuff like remote controls?

Most AA battery powered devices are designed to accept a fairly wide range of voltages because the raw voltage from a battery is inherently variable, and 1.2 is almost certainly within tolerance (depending on how reputable the manufacturer is, there’s a good chance the engineers specifically tested it with rechargeable batteries to ensure they work, since it would be a really bad customer experience if they didn’t work) Besides, for high power stuff, usually it’s the current rating that matters, specifically, something called equivalent series resistance AKA how much the voltage drops when under load. High quality NiMH batteries can deliver a lot more current than alkalines so depending on what kind of load you have on it, the voltage of a NiMH might even be higher than alkaline when the voltage drop is factored in. I’ve personally never had any issues with NiMH’s in high draw devices.