That's looking at things purely from a UK/EU perspective. There are many, many countries where their citizens can stay in UK for up to 6 months in one but only allow UK citizens to stay for a maximum of 3 months without a visa at a time and they include Commonwealth countries.
If UK announced EU citizens can only stay 3 months, would they do the same for US citizens, for example?
I feel we are falling into the trap of thinking UK is being punished for some reason when the same rules apply to every country that can travel visa free.