On the contrary, they enhance it. They permit a judge to impose a greater penalty in precisely those cases which warrant it.
There can be no argument that legislature has plenary authority to define crimes and set the punishment for crimes, subject only to the limits imposed by the Constitution. There is no usurpation of judicial authority when the legislature, in the exercise of this power, provides instruction to judges on the factors which they are to consider when fixing sentence. Such usurpation (IMO) only occurs when legislatures strip judges of discretion entirely (mandatory life sentences, “three strike rules”, and so forth); in my opinion such legislation violates both due process and the Cruel and Unusual Punishments clause (Harmelin v. Michigan notwithstanding).
However, no hate crime statute I have seen interferes with judicial discretion. In most cases all the effect of a special finding of bias motivation does is allow the judge to impose a greater sentence than otherwise would be permissible for the underlying crime. In some situations, such a finding may also increase the minimum sentence (usually, in cases where a special finding of bias motivation results in the offense being upgraded from a misdemeanor to a felony). In addition, the trial judge is free to entertain a motion for a directed verdict on the special question of bias motivation before the question is even sent to the jury, if the prosecution has failed to make a decent case for it. (Not all states require a special finding, leaving the determination of bias to the bench alone or requiring a separate pleading in the indictment. I am basing my comments on Wisconsin’s statute, which is footnoted in the Mitchell case I cited earlier in the thread).
For what it’s worth, I don’t think hate crime laws offend the spirit of the Constitution. If anything, they reinforce it. The Constitution, especially as amended by the First and Fourteenth Amendments, embodies the principle of a pluralistic society. Hate crimes are offenses against pluralism, and prohibiting them seems very much in keeping with promoting a pluralistic society.