At the risk of oversimplifying, the removal of trees stops water from being taken from deep under the soil. Water from irrigation soaks through the soil, which raises the water table (the surface of the underground water). Salt rises with the surface of the water (salt water is lighter than freshwater), and deposits on the soil surface. The salt stops plants from growing, drastically reducing the productivity of the land.

Salination is a local problem caused by bad management of resources. The main resource that a farmer has is his land. If that resource is mismanaged, it can be damaged, and that is what many farmers are doing - damaging their asset. Any other businessman who damaged his asset would receive little sympathy from the public, and even less in government subsidies.

Salination also causes externalities - the salt frequently runs off into the waterways making it less useful for irrigation, drinking, or household use. Clearly it is a form of pollution. Few other industries would receive government grants to help them reduce their pollution production.

Strangely though, the farming lobby groups have been very successful at getting subsidies for their self caused problems, and their pollution of other people's resources.