Can a US university force a foreign student to obtain health insurance from a specific provider?
According to the international student health insurance page at the Colorado School of Mines, which also requires international students to enroll in the school insurance plan, it is legal. 

The legal rationale they offer is:


It's not considered discrimination to have different policies for U.S. citizen vs. international students, so long as the requirements are the same for all international students regardless of country of origin:


  "The University may impose such a requirement on all students who are not United States citizens. Such a requirement is not regarded as discrimination on the basis of national origin by the courts. See Espinoza vs. Farah, 6 FEP 931 (November 19, 1973). This is true so long as no distinction is made between citizens of different foreign countries” (Joseph Goldhammer, Resident Legal Counsel, Office of the Attorney General, Dept. of Law, State of Colorado, regarding Foreign Student Health Insurance).

Federal immigration law itself imposes certain health insurance or other requirements on foreign students: 


  Most students come to U.S. institutions under an “F” visa. Federal immigration law requires that this subclass of non-immigrants provide proof of their financial ability to support themselves and the cost of their education. These requirements have not been found to violate the equal protection amendment to the constitution. (Anwo vs. Immigration and Naturalization Service., 607 F.2d 435 D.C. Cir 1979). Colorado School of Mines considers medical insurance an integral part of the students ability to support themselves. Federal immigration law also requires that exchange visitors and their accompanying spouse/dependent(s) coming to the United States on a "J" visa carry medical insurance. ( 22 CFR 515.14)


(Note that J-1 visa holders are required to have health insurance meeting certain minimum requirements, not just any health insurance.)
Courts have recognized that specifically mandating health insurance for foreign students is not discriminatory:


  Equal Protection Analysis/14th Amendment to the Constitution: The U.S. constitution requires that "no state shall... deny to any person within its jurisdiction the equal protection of the law." Is requiring health insurance of international students discriminatory? Does it violate those students' constitutional equal protection rights? Case law differentiates the legal rights of U.S. citizens and permanent residents from the legal rights of non-immigrant aliens, those persons who are in the United States for a short period of time and for a particular purpose. Courts have recognized that requiring health insurance protects the students' ability to exist in the community, realizing the high costs of health care in this country (Ahmed v. University of Toledo, 822 F.2d 26 6th Cir. 1987). Further, courts have acknowledged that requiring health insurance is consistent with, and supports the purposes of, federal immigration law. “International students do not have a constitutional right to attend American universities without complying with the institutions' reasonable regulations.” (Plyler v. Doe, 457 U.S. 202, 1982).