Shoplifting is treated as a minor crime unless it is by a 'professional' or repeated many times.
As I just said in another post, it takes 2 to 4 hours for a police officer to fill in the paper work after an arrest (a shoplifter has to be arrested to take a case to court) A lifetime ban from the shop in question is a lot more effective , quicker and saves a lot of time.
Do you really expect or want thousands of hours of police time to be spent processing shoplifters,when most shoplifting involves £20 worth of goods or less?
Even if a shoplifter is taken to court the normal result is a, fixed penalty notice, which is not a criminal conviction anyway and does not nearly cover the cost of the arrest and paperwork.