It comes down to each countries individual consumer protection laws . Here in Australia they cannot just void a warranty, if the warranty claim can be a result of the non factory accessory then they have the right to refuse the warranty but cannot refuse any warranty that has nothing to do with the installed accessory.
There are grey areas especially when you add exhaust systems, if you had engine damage such as a burnt valve or such then they would have some grounds to blame the cat delete system. Their factory cat delete system has been tested by them and a suitable map is installed that they have been able to test and sign off on being ok.
Even if you reflashed the engine map after installing a non genuine cat delete they will not honour that because they had no control over it.
The consumer protection laws are there to keep manufacturers honest but they do have rights to deny claims if they believe it contributes to the fault when you decide to use non genuine accessories.
