Do any countries' emissions regulations require manufacturers to actively try to prevent illegal modifications? It seems to me that wouldn't be the manufacturer's problem, and I can't recall hearing of a legal case where it was.
This seems more likely to be warranty-related since it's very easy to break mechanical parts by adjusting things like boost pressure and ignition timing.
Modifying modern cars for high performance impacts the emissions systems. In many cases the ECU modifications will actively defeat certain emission control measures to allow the higher power output without check engine lights, for example.
Showing a good faith effort that you've designed the emission compliance systems to be reliable and hard to defeat is part of the process.
There's a similar thing that happens with WiFi access points: You have to show that you've made an effort to prevent the end user from trivially modifying it for higher power output or other changes that would deviate from the way it was compliance tested.
This seems more likely to be warranty-related since it's very easy to break mechanical parts by adjusting things like boost pressure and ignition timing.