What's Going On With Supermarket Chocolate Security In The UK?

Recently, UK shoppers noticed some plastic security boxes surrounding chocolate bars in their local supermarkets. 

Sainsbury’s has said these anti-theft measures are being applied to “products which are regularly targeted” by thieves. Tesco has covered some of its chocolate products with a sliding plastic shield, which makes the bars harder to slip out.

This is true in my local Tesco, where a £2.10 Dairy Milk is kept behind a transparent barrier.

Why is chocolate being kept in security boxes in supermarkets?