Tuning a 2011 Mustang 5.0?
There's a counter on the pcm/ecm/ecu that gives historical information. Meaning if you re-calibrate the computer it pops up when the dealership scans the obd2 system.
Would the tune be harmful? On this particular engine (because it's running high compression and direct injection) it's likely that the tune makes the car run richer not leaner so it's probably not a problem. But that's up to you to find out through the manufacturer, not me. If it's running leaner you increase the possibility of lean seizing the engine.
The reason they "require a tuner" is probably 1)because they want to sell them to you and 2)because they don't want to affect reliability (as described above). Steeda isn't one of ford's main aftermarket partners so the last thing they want to do is have their parts damage engines and have the ford dealerships potentially pay for it.
If you want to maintain the warranty DO NOT add anything to the car for as long as you want to maintain that warranty. If you want performance parts only buy the FORD RACING parts and have them installed at the dealership so that maintain your factory warranty.