I worked at a few 7-Elevens in my hometown, and these are from personal experience.
The reason that 7-Eleven has locks on their doors is two-fold:
1) Because if there is a violent situation, the employees must be able to lock the doors to prevent violent people from entering the store. The store is also locked after robberies so that the robber cannot return before the police can show up.
2) 7-Elevens may be open 24/7, but there is one exception other than violent situations, and that's Remembrance Day (November 11th), during which it is required that the store be closed for a minimum of three hours. During that time the staff are usually still in the store cleaning.
Another thing that none of my co-workers had ever noticed until I pointed it out to them during my training is that the 'n' in the word eleven in all of the store's logos is always lower-case, while all the other letters are upper-case (7-ELEVEn). We assumed this was done by some graphic artist for aesthetic considerations.
*Please note that the preceeding applies to Canada, but not necessarily any other countries.