Considering they didn't generate the keys out of nowhere, their origin was from a legit source, be it give away or special price from the developer. So as far as it's theirs, what's wrong in selling them for whatever the price?
Because depending on where and how the keys were generated this could directly impact the profit margins for The Witcher 3.
Even if the keys are legit, they were generated for a different vendor. They could be the nvidia promo keys, they might be keys sold in a different region with a lower price expectancy. So they might be "cheap" keys that don't make a lot of money for CDPR.
CDPR obviously doesn't want everyone to get those. Notice how no one else is selling lower than GOG? That is by design. GOG gets them the highest profit margin per sale. Everyone selling in the US is pricing about the same and this is all worked out on the business side of things.
So now here comes GMG with these keys of unknown origin or cost and they are selling them cheaper than everyone else. Know what happens? Gamers cancel their preorders at GOG and other sites to buy from GMG. This then cuts into the profit margins of everyone involved (including GMG if they are making barely anything or even taking a loss).
The key piece of information missing is who supplied GMG with the keys, and it will make all the difference in this matter.