If your bag doesn't fit in the guages then it's too big as the guages are actually larger that the allowed bag. So if it fits it's free. If it doesn't then you'll need to pay for it as it's definitely too big. Incentives or not, if the bag is too big then you're not entitled to carry it on for free so I don't see any reasonable grounds for a complaint.
If you're talking about offloads due to too many bags, then there is no charge for this so I can't imagine there's any incentive.