It's an entirely bogus reasoning and there's no way to claim otherwise. The same device is shipping a myriad other software which can be classified into:
A. Outright copyright violations of proprietary software (i.e. "abandonware")
B. Legally distributed software (e.g. some of the BIOSes of some of the add-on cards), which their permissive licenses generally allow as long as you keep the attribution, which just from the screenshots I can see they did.
C. The main system ROM BIOS, for which some unfathomable reason they _specifically_ decided to remove the attribution (which is technically in violation of both the spirit and the letter of the GPL).
So to claim that they had to violate the copyright of C for "protection" is as bogus as it gets. They have already set themselves up for a cathedral-sized _non-bogus_ DMCA claim, and violating the copyright of the GPL'd BIOS is just the frosting in comparison.
My wild guess: branding. They just disliked not to see their company name first on power on. Plus the general tendency to treat the GPL as a "guideline".