Either it’s because of safety, in which case fair enough, or spite, which isn’t.
It’s just rules. We will automatically transfer from being a country internal to the EU to being a country external to the EU so a different set of COVID control protocols will apply to us.
Those rules already exist and apply differently to - say - Brazil and France as we speak. We are just stepping outside the boundary overnight.
It can’t be “spite” that once we are outside the EU the EU will probably treat us as though we are outside of t EU.
In fact, there’s still the possibility that they will be generous and give us a staged arrangement.