I'm from Minnesota so this is just from memory. I checked the u of mn website and I was almost correct.
Reciprocity agreement with : WI, ND, SD, and IA
Nebraska, Kansas, Michigan, Missouri are also in a tuition reduction agreement with minnesota through the midwest student exchange program.
sorry for the previously confusing information.
Reciprocity and the midwest exchange program are agreements between schools to allow enrollment of reciprocal state students for close to in-state price, so I would simply guess that Illinois decided not to take part in it.
There aren't any disadvantages to reciprocity. You pay close to in state tuition but you are not forced to attend or anything.
Remember though that it applies for undergraduate admissions and I am not quite sure whether that also applies for professional/grad schools...