Requiring workers to be punctual is not "a government policy."
In any case, in your larger Wal-Mart stores, folks work shifts of different durations that start and stop at varying times.
The "service" problem is one of having sufficient people there at appropriate times to move stock into place, or man cashregisters.
Although the workers seem to be able to handle the schedule, supervisors have a tendency to want to buck the schedules and knock folks into what they view as "punctuality" by means of incremental changes of worker schedule that tend to be invisible to higher levels of management.
Wal-Mart will end up with the same problems the post office (in this or any other country) has always had.