[Bigun]

Congress can't tell the Court how to do its job, only the Constitution can.

Perhaps not, but the Congress clearly can put whatever they wish off limits to the courts! See excert from the Constitution below:

Article III, Section 2, Clause 2:

In all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party, the supreme Court shall have original Jurisdiction. In all the other Cases before mentioned, the supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make.

[Grut]

Art.3 Sec 2 C2 gives the Congress control over the Court's jurisdiction, i.e., what types of cases the Court can hear. It doesn't give the Congress any control of the Court's deliberations, which is what the proposed law would assert.

I think the idea of restricting the Court to American law is a good one; I just don't think it can be done without a Constitutional amendment.