Why don't the police confiscate the car of any driver sitting outside a shop/pub etc with the engine running whilst waiting for his partner/friend to shop/finish a pint etc?
Are you not a car owner, are you without a driving licence, are you jealous of car owners/ drivers? Are you bitter that you come into one of the above groups?
I would imagine because:
1) They don't have the power to do so - I'm not aware that what you describe is an offence!
2) If they did where would they put them all - the police do not have a never ending supply of car pounds!
3) It is a question of priorities - ask any driver who has been given a speeding ticket - they will tell you that the police should be spending their time catching murderers and rapists not persecuting the 'innocent' motorist.
4) There are not enough Policemen/women on the street as it is without tieing them all up arranging the recovery of vehicles stopped as you describe.
I would be interested to hear why someone would have such a downer on motorists - perhaps you live over a shop which is often visited by motorists waiting for their partners/friends as you describe.
If so perhaps it would be easier for you to move!
Here in Toronto it is an offence to leave a car idling for more than three minutes, for air polution reasons, but I have never heard of anyone being charged. I suspect the police have better things to do with their time. Why do you find this so offensive?