"Reasonable expectation of privacy" is a term of art in US law, and has been defined in countless cases. If the FBI wants to come in and search your HD for your browsing history, yeah, you win: they need a warrant. It's private. In your house. If, OTOH, they go to Macrumors and say show a list of who browsed the thread "Google photos..." you're SOL. But if we were talking about an email exchange we both had over Gmail, that would probably be a different result.
I think there is a difference between content that has been published to the public (like forum posts) and private information (say, credit card information, private email messages or login credentials).
And likewise, the cops can follow you around in public. From Starbucks to Starbucks. And Starbucks could do that as well, say if they wanted to do market research to see how fast you drank that vente.
In most jurisdictions I'm aware of, there are laws against stalking and harassment

Starbucks tracks you via different information, though (e. g. your credit or loyalty card). And if you don't want to be tracked, there is a simple and obvious solution. On the web with most trackers, that's no longer an option (e. g. via undeletable evercookies and several trackers implanted in each ad that follow you around the web even after you leave the page). Likewise, if Google Photos continues to track you after you have deleted the app, that's a problem.
But you are right in that they "can't do whatever they want with it" depending on what "it" is. Just as we are subject to laws and implied or explicit agreements about what is done with data, so is Google.
I think you touch on an important point, namely that laws and regulations are lagging behind with technological development. People have not come to grips about creating a balance between expectations of privacy and the ability of using free services (which in most cases need to monetize your information to run the services). And I think that's why you can't divorce the discussion of Google Photos from Google's business model — which is really unfortunate, because Adobe and Apple would kill for the automatic indexing capabilities.
... we, OTOH, are often clueless. Which is why you should know what you're getting into, specifically, when you use a service like Google. If someone uses Google, especially free services, without that due diligence I find it hard to feel sympathy with their loss of privacy.
Well, yes and no. I deliberately try to avoid using Gmail and other Google services for that reason (although I do use Google search). Try to avoid Google and you'll see how hard that actually is. But I don't think Google should have a
carte blanche or that it's really clear
how the data they have on you is used. Probably that's Google's best guarded secret, though, and they are not inclined to divulge anything.