What I think is wrong is the assumption that any parent that may try to discipline their child by smacking is being abusive to their children - not all parents who smack their children are abusers, although most of you seem to think they are. There is a difference between smacking (which is actually done relatively gently) and beating.
Also, I am sure most parents are at their strictest when they are out as this is where the most dangers are present, ie, wondering off, running into the road, pulling something unsafe on their heads.
If a child is too young to remember what naughty things they did when they were out at the shops it would be mean to then say "you can't have x,y,z because today you did this" at a later stage during the day. They won't remember. They won't have a clue what you are talking about.
The other alternative is that you completely forget about the consequence you threatened and then the child is not getting any consistency or discipline at all.
For a consequence to be effective it should be immediate. For all of you who have never smacked the sequence goes something like this:
First warning - don't do that + explanation.
Second warning - I told you not to do that + explanation and if you do it again you will get a smack (ie, consequence)
Third time - mummy told you not to do that - enforce the consequence, ie, a smack on the back of the legs not too hard but just enough to shock them into behaving.
You will note that a smack is NOT the first option. Also, if I were to implement it, it would not be my first choice of consquence. If you are at home you have more options available.