I think you should fight to get them for nothing. You have spent time getting a 4 wheel alignment and quite frankly the dealer is crap for asking you to get it done, they know full well there is no adjustment so it is NOT a service item unlike the front wheels. I suspect they also know there was a problem as it is very common.
You will have to take time to drop the car off and have the new tyres fitted, all this costs you time and money.
This attitude from dealers really annoys me, they should be apologising for the inconvenience you have experienced.
To me it is a matter of safety, as I understand it the Mitchelen tyres are heavy load versions with stronger side walls, which prevent the tyre deforming. The tyres currently fitted are probably deforming under load and wearing, it also makes you wonder whether the profile of the tyre when it is deformed affects the grip.
Just to show you the level of service from Fiat, both Citroen & Peugeot recalled the C8 & 807 in July over safety concerns. It took Fiat until this month to recall the cars for the same faults. These are safety issues and there is no excuse for the delay.
EDIT: By the way the comments weren't aimed at you NumanR, it is normal practice as you said but I think this dealer could do better because of the hassle factor.