How are they disenfranchising Republican voters? They weren’t deprived of their voting rights, and crossover voting is perfectly legal in Michigan.

Was it partisan? Yeah, looks that way. Was it unethical? Maybe, but it wasn’t some sort of electoral fraud. If the Michigan GOP doesn’t like it, then they should work at having the open primary voting laws changed. They may not want to do that however, as they can use the option to their benefit as well.

Michigan Presidential Primary Facts and Statistics (PDF)
Provides legislative history going back to 1912.