Didn't some Linux maintainers had very clear legal words for NVidia when they tried to make a kernel HAL/ABI?
Also as far as I remember (I might be wrong) the google HAL isn't a abstraction/redirection in the low level kernel module layer it lives on a higher abstraction level and was meant to make the amount of ways braking less but never planed to add abstractions on top of which you could build a graphics driver or CPU support (which doesn't only affect drivers but potential also kernel compiler time conditional compilations).
When asked about it most kernel developers point to closed-door negotiations that are being kept confidential. Usually also saying "trust us, we know what we are doing, we'll get more source opened our way".
Those who actually try to enforce GPL publically are ignored or shunned, e.g. Harald Welte/LaForge of gpl-violations.org