Tire Shop Michelin Tires Story

[Gimpy]

Last week I noticed a couple rear tires had started to crack and would need to be replaced with new tires so I drove down to the tire shop and had tires installed. When I went to pick it up they had replaced the two front tires with new Michelin's and rear outer tires with new Michelins but the rear inner tires where still originals. There does not appear to be dry rot/cracking on the side walls but my question is is this acceptable to do because the tread wear is obviously not the same on the outer and inner tires. Originally what I was thinking was replacing all rear tires but I guess they saw fit not to.

Any help on this question is much appreciated!