Administrative law judges already have far more discretion than most civil servants or other judges. Politicizing their selection means they become more loyalty to pursue the Trump admin's goals.
Actual administrative law judge on making the position appointee-only: “[Trump] didn’t have to do any of this. This is all bogus, and it’s to take control over the judiciary. This is a total assault on due process for the American people." beltwaybreakfast.com/courts/2018/07… 
One scenario for the new administrative law judge appointment power: hire lots of positions that have to be responsive to political bosses. Then lay off those with existing civil service protection b/c there is not sufficient work for everyone:
