Frankly, I never felt the act was anything but a violation of Picard. Ultimately though, the perspective of the episode is that it is forgivable as the violation is perpetrated by extinct people, & Picard's willingness to let it in & treat it as something of value makes it seem like less of an assault
Although that's pretty much how Stockholm Syndrome works too
Edit: Wow, now that episode takes on a whole new angle for me, Picard suffering Stockholm Syndrome. I don't care what anyone says, that's some smart writing
I do think that point that Picard did not have a say in his mind being probed is a good point that people have brought up. Maybe the writers didn't think about it?
I don't think that episode works if Picard knew it was just a mind meld type of incident. He had to truly become one of these people in order to experience them.
So while it may have been a violation technically, I am willing to give the writers and the people in Picard's experience, some slack because I think it is a moving episode.