Any modification to any vehicle takes it outside of the parameters in which it was built. Car construction today is so much tighter than it ever was in the past. So removing the tightness to any specification can have adverse affects on the car, no matter how small. The makers (all of them, not just GM) are only protecting themselves against the actions of others, which often in the past have led to extremes such as lawsuits and other actions which are not very good for the company, or all it's customers.
If a problem were too occur due to a change, not saying it will, BUT if it did why should the maker be held responsible? They shouldn't, so that's why the specific wording in the warranty contract.