Its not even as though they're slagging them off - they're making performance comparisons, which is what manufacturers do everyday.
If, as a manufacurer, you aren't allowed to make comparisons against another manufacturers product, be it on the grounds of performance, equipment or quality, how can your put a product in to context?
I think you're over-reacting a little here. I have friends who drive ZR's and ZS's and - like it or loath it - they are genuinely quick cars.
I have personally driven a ZT 190, and whilst its never going to set the road alight (hence the need for the V8 version) it is a quick car.
If there was a ZT 190 and Mk4 2.0 GTi lined up against one another on the strip my money would be on the MG, and so would anyone else's that's actually driven one.