The magnet kits are out there for people to buy anyway. Though not everyone wants to have to pop down the hobbycraft store or go online to buy them. But it is all free money if they did sell them.
A pack of 100 costs about £1.50ish to buy http://www.amazon.co.uk/Big-Bargain-100pcs-Neodymium-Magnets/dp/B00AAWEH9I/ref=sr_1_1?ie=UTF8&qid=1404559226&sr=8-1&keywords=magcraft is the normal ones I get.
But probably costs about 50p for them to get hold of to sell. Also with GWs general mark up + because of the convenience of being able to buy them in store with your models they could probably sell them for about £3 for 100. Also selling Magnetising kits such as a special drill and brush to clean out the holes they would get their return on it easy enough.