It isn't. You said you can not see why they don't do the same. Gromit says it's a different situation, milk and beer. That there is no over-supply of beer such that market forces force the price down. It's a deliberate policy to buy at the wholesale price and take the hit. Whilst for milk the retailer is in control since excess global (or national) production can not be sold elsewhere for a better price so the producer gets screwed.
Where these excessive situations occurs government intervention should be to correct the situation not let it fester. The EU decided a large supply was worth ensuring, thus the resulting problem with excess. If policy creates the excess supply then the means to buy up and use it at decent prices should exist too. But that requires forward thinking and political will.