So they're being paid not just for not using this power at that moment, but being part of a system of "demand response" that lets the grid plan ahead and modulate demand.
They also do this with people's AC and various large industrial energy users.
But, time-of-use rates are a currently underutilized idea, and with greater computerisation, you can get more and more value from this on short-timescales.
You want to start with the low hanging fruit first, and EV chargers are that easy win as they are being installed now.