Also, Sunday trading laws prevent retailers from opening all day on Sundays.
They have to close at 10:00pm Saturday, open between about 11:00am to about 5:00pm on Sunday and then re-open on Monday at 08:00am. Therefore they lock the doors between these times.
Of course, I know you were posting this for a laugh but on this occasion, you asked a question with a proper answer! Perhaps Viz magazine doesn't post these answers...