I agree that everyone played nice it wouldn't be a problem.
However malls are IMHO a particular problem, even in the UK. I've seen multiple posts on UK fora of people saying how they save up, go to the mall, go shopping leaving their car until they've finished. When pointed out that this is not "good" etiquette, there are mixed reactions. Some saying "fair enough", some saying "I paid for it", some say "I'll just flatten the battery more and set it to 100%, that will give me more time!". By requiring physical presence you dissuade the abandonment and "nickel and diming".
OK it's not as convenient, as it means you have to be in the car. But it's still free and unlimited to the extent owners are willing to sit in their cars. The downside to "good citizens" is if the time to get a coffee and return to your car is > 10 minutes, you must re-initiate the session on return. The inconvenience would be the difference between 10 minutes and your actual "coffee fetch" trip time, so lets say 10 minutes max. Would that be better than the inconvenience of arriving and all the stalls being blocked with the owners all AWOL?
In fact Tesla could be selective on this. With some Superchargers where congestion/abandonment is a problem requiring presence, others where it isn't don't.
An added bonus to this would be each of these non presence session time outs can be counted, and easily flagged to Tesla to address persistent "abandoners".
What possible reason would a property owner have to allow a Supercharger to be installed on their property if Tesla mandated that owners stay in their cars? Do you really think they are allowing them to be installed at their malls and restaurants out of the good of their hearts?