I don't know. I mean I can see how people would consider it rude/stange/creepy if a stranger took pictures of them and then posted them online without their knowledge. At the same time, I think it's kind of a ridiculous expectation that you can do anything in a very crowded touristy location...especially something that people often are drawn to (like proposals, weddings, etc) and think that no one will notice, photograph you, or share your picture. Of course, I would call someone who decided to follow me around and photograph me rude and creepy. I might even get mad. But, if I was getting engaged and that concerned about my privacy, I probably wouldn't choose to do it at the eiffel tower. And I would also assume that the proposer knows the proposee well enough to understand her feelings on the subject.
My conclusion is this: This woman noticed something and took a picture. I'm sure she wasn't purposely seeking them out to invade their privacy, so it's likely that, as public proposals often are, it was noticible. I'm also sure she thinks she's doing something nice by posting the picture and trying to find the couple. I also think it's a decent assumption that this couple isn't exactly expecting that their moment will be completely private and weren't seeking that out. I think the whole thing is innocent.
I don't expect that my image is never snapped when I'm out in public. Even if someone isn't taking a picture of me, I can still end up in a picture that ends up online and goes viral. I once ended up in the background of a picture that was used over and over again to advertise something I was working on. The picture was candid and I wasn't the subject, but I was very recognizable. I was also at a public event. That's just part of the risk we take when we go outside.
And while not the same as being the unknowing subject of the picture, I do think that if you want to ensure that your private moment is kept private, that you do it privately. By being out in public*, you are communicating to those around you that you are ok with the attention.
*And that's being out in public sharing what some would consider a private moment...like getting engaged.