These are "spy"marks, not watermarks:

https://brand.io/article/spymarks/

We need to inform everyone that this technology can encode database identifiers and enough entropy to uniquely identify you as an author (or downloader).

This extends to other forms of media as well.

It's worth nothing that this IS Google.

Given that it's an ad company, they want something to connect what you're doing to the rest of their data, and they couldn't possibly keep the old image search results there if they want people to use this.

It's presumably so they can rate-limit people who are trying to reverse-engineer or otherwise strip it (e.g. iteratively tweaking until it stops getting detected)

Yeah I've been wanting to run a ton of Wikipedia images through the detector to see if fakes snuck in. Doesn't seem to be a practical way to do this. Even Open AIs tool has a low rate limit

The EULA you have to accept hints that they are afraid you are going to abuse it to remove synthID - e.g, set up and edit and check loop

Any access to a watermark detector can be used to remove the watermark. Presumably they want to be able to track who is doing that.

I've never seen it mentioned anywhere so I will just here complain that Google also removed the ability to paste images into Google Image search some years ago for no apparent reason. It worked great and I used it all the time.

OpenAI's tool is worse though, because it doesn't detect SynthID from non-OpenAI models. I just tried it with various images created by Imagen / Nano Banana.

It's deliberate so that you can't find a way to bypass it.