The issue is that you are filtering the recordset based on the posted value and failing validation removes the posted value.
The solution is to set the default value to the persisted form value on line 98 like:
$colname_rs_validata = WA_getSavedFormValue("persist_data","tipo_punto_vendita_reg")?WA_getSavedFormValue("persist_data","tipo_punto_vendita_reg"):"-1";
That should allow you to drag from bindings.