Most tuners have to do it because of clientele. If your 1.9TDI gets beaten by your buddy's 1.9TDI, your tuner's ass's going to itch from all the cursing.
No, it is done because of lack of knowledge and doing a shit job.
On Diesel you only have to set a reasonable EGT and IAT limit, that's all there is to it. And by setting correct EOI (most diesel tuners don't even know what that means), you will have better performance everywhere and lower EGT.
Correctly tuned there will not be any difference between the cars performance.
Except one will blow up in extreme conditions and the other one will limit the output at that point and stay together.
This thinking that somehow you need a tune for power, economy etc is complete bullshit done to sell more of the same shit.
ECU has full 3D mapping. ME7 has ways to limit requested load during too much knock intervention/low ignition efficiency and the 1.8T variant even has real closed loop EGT control.
The only excuse for having non-fullthrottle safe tunes is laziness and ignorance, which this thread reeks of.