I think if they had literally "rigged the primaries" in the sense of interfering with the voting process or doctoring results, that would be a problem. But a party can do whatever it wants in terms of allocating delegates and setting rules for how those delegates and party officials go about selecting nominees. Of course if they're perceived as having subverted the will of their voters, they face some backlash - especially when their nominee goes on to lose the general election...