I am not sure the supermarkets are being 'greedy'.
As a business, they have a duty to their shareholders to make as much profit as possible.
There is an age-old practice in retailing called 'loss-leading', where a business will sell a popular commodity at a loss in order to draw incustomers, who will then by high-profit stock which will offset the loss incurred.
That has gone on for a very long time - all retailers do it, and the major supermarket chains have the clout to be able to really hammer down the price of their chosen loss-leaders - and a staple like milk is an obvious product to choose for this practice.
I suspect that they have done some rapid opinion-testing, and found that the customer will bear the additional cost, and their minor profit loss is offset by a mass of free advertising through the media as their campaign is rolled out.
Morrisons et al do not get to their level by not understanding how retailing - and image - work.