A possible answer? Under a system of checks and balances, a judicial branch is supposed to be a check on both the legislative and executive branches.
It's hard for the judicial branch to be a check on the legislative branch if the legislative branch can reduce judicial salaries each time the legislative branch disagrees with a judicial opinion.
That’s a rather roundabout explanation, and anyway has nothing to do with the actual case. Ifsoever Congress attempted to control the judiciary in that manner I think it might run afoul of the ban on bills of attainder. They could only really punish classes of judges, and that would be an inefficient way to get them back for specific rulings.
Judges couldn’t forestall Congress from reducing salaries on whole classes of judges because they may be punitive. Congress does, after all, hold the purse strings. That is their check (or balance); the judiciary can’t take it away.
By the way, if Congress wants to punish judges for bad rulings the correct route is impeachment. It was a sad day for the Republic whenever it was we decided to against political impeachment.