legality infoblox appliance

I recently followed a small talk about the appliance Infoblox
(a dhcp/dns/etc appliance) and talked to one of their technical people
since this product was running linux and I was wondering about the
source code...
now he claimed that since they didn't -add- anything to the kernel
source, but only removed code from it, they didn't have to provide any
source code for this. But as far as I know it's about changes to the
code, not additions. 
Could anyone tell me which of this would be true ?
they don't provide any code as of yet, but since atleast some of their
customers require legality, they seem to be willing.


Thanks,

Sebastian