Because they placed the content on a public website.
> What happens if the publisher can’t grant this license?
Then it's unlikely that the content is something they themselves have the rights to broadcast on their website.
The publisher using stock images does have the right to transmit those images to visitors. Those visitors don't have the right to incorporate the images into their own public work that they rebroadcast. But Meta isn't asking for that; they want consent from the business to scrape. The subsequent copyright violation is then a matter between Meta and the vendor of the stock images.
Imagine someone asks you: see that guy across the street? Do I have your consent to search his pockets? If you say yes, what does it mean? Precisely nothing.
Suppose you are asked: do I have your consent to search the pockets of everyone in this neighborhood? If you say yes, it still means nothing with regard to the guy across the street, who has not given any constent, but you've consented to your pockets being searched.
I think if you have some kind of authority over the place where this happens and/or if you facilitate an illegal action then you could be breaking the law.
If you signed some agreement whereby you pledge to protect licensed images from AI scraping bots, then signing off on a consent to Meta would breach that agreement.
There is no such agreement. Licensees of images are understood to be broadcasting them to the entire world, including anonymous visitors.
I don't think this is a very good argument. For instance, if I have a company, called 'tickle-cats', I may expect regular customers to go and tickle a cat for, say, 1 cent, as cost. So I would expect regular people to visit my homepage.
I may not necessarily expect or want to have bots, scrapers and what not, per se. I may have this by extension - possibly, e. g. to rank a website more highly. But bots and so forth are not my real target audience to begin with (and some are just malicious, trying to disrupt the business); some company owners may want to not have to do anything with them (bots, crawlers etc...). That's just one example of many more; I don't think the distinction "public versus private" is the only one that can be applied here.
However, it is something that happens, and when you license copyrighted material to someone such that they have permission to serve it to visitors on their website, they are not automatically required to be a cop guarding your material against scraping.
There is an understanding that public-facing content gets scraped and used for AI training and whatever else. Someone licensing content into that role is accepting that reality.
Also, anyone in their right mind granting consent to Meta for scraping will still block them from doing that. That action speaks louder that the consent. If some content licensor accuses you of giving Meta consent to steal their material, you can point to your implemented physical defense against that and argue that the only way it could be happening is if Meta are circumventing your utmost efforts block scraping.
If I record the songs playing & they sell bootleg CDs, I don't have a defence of "but it was played in public, so all copyright was waived". That's not how copyright works.
If the musician gives someone consent to record (plus subsequently use the material in various ways) then in fact they do waive some of the copyright with regard to any of their original songs. The consented party has been given an implicit license to the material within the bounds expressed in the consent.
The status of the licensed songs is not affected whatsoever, and the musician has made no legal representation in regard to that. The musician is paying the royalty fees to PRS and doing everything they are supposed to.
The musician can say, "you have my consent to record and use the recordings, but be aware that some of the songs are copyrighted by a third party and I'm using them under license. Nothing I say gives you right over those songs."