I completely disagree with what they are doing for a few reasons:

These sites honestly aren't all that shady.

They are taking advantage of the fact that we live in a global economy. It's not like they found a big bag of game CD keys on the street like some kind of cartoon villain. All (or at least the vast majority) of the keys distributed by these sites are purchased from parts of the world with regional pricing models that "force" publishers to drastically lower their prices, and by trying to make a profit from these markets they are opening up themselves to the possibility of this happening.

If companies like Ubisoft want to avoid being exploited by a system that they're playing in to make money, maybe they should stop selling their games in these regions. It's genuinely not the consumers fault that the internet exists regardless of how they buy a perfectly legitimate game key.

Alternately, many of these keys are obtained from promotions, such as buying a video card and getting games for free with it, that are simply unclaimed or posted by people who simply would rather have $10 than play the game given to them for free. These keys have all been paid for by someone at some point in order for them to have even been distributed and are absolutely not shady. The buyer is taking all the risk in this transaction, and publishers should not be able to decide how keys they agreed to print are being used since they've been paid for them.

Large sites selling keys that have been illegally obtained (credit card fraud, etc.) are never the ones who committed the crime.

Operating a site like G2A exposes them to a ton of risk. Much like a pawn shop, they have absolutely no idea if something is stolen when they agree to buy it, and in the event it is, they take the hit.

I don't have metrics on this, but I assume theft of video game CD keys is an extremely low volume of the keys sold by these sites. If they ever buy a batch of stolen keys, you can bet your fucking ass they are never going to buy more from the same person and they could easily turn them into the authorities if they tried again since they would have their payment details. This makes it extremely difficult for selling stolen keys to be a sustainable venture.

If Ubisoft actually communicated with these sites instead of treating them like criminals (which, as stated above, they really aren't), this problem could actually be easily fixed long before little Johnny buys a key off G2A that is already banned.

Bottom line: Publishers are butthurt that they're not making the money they want to based on their inflated regional pricing models, and this is the only form of reprise they have.

What they're doing is literally equivalent to stores not honoring a Gift Card you bought for 50% of Craigslist, and it is 100% bullshit. Every CD key ever distributed for a video game is signed off on by the publisher.