I heard about this a while back
It sounds a bit like diesel to me - they told everyone to buy one and then once there was a nice little node to tap into they changed the rules
With the diesel the problem was, politicians did not bother to read the small print (don't we all?).
The headlines said diesels produced less CO2 the small print said they produced more NOx (a greenhouse gas) and soot (unburned fuel).
The tax was increased in line with other fuels to reduce the generation of greenhouse gases.
In the case of EV's, I think everyone understood the no road tax situation was only temporary.
A government's only income is from taxation, road vehicles are a cost to the state, new roads, repairs, police, ambulance etc.
Without tax on vehicles the same money would have to be generated from say income tax, it has to come from somewhere, we have already maxed out on the state's credit card!